Why does the King James Bible say "I shall not want" and then "I will fear no evil."

I have a question relating to the use of "shall" in the Bible. Before asking this question I read the entire Wikipedia article on Shall and will, and have ended up more confused than when I began. There have been some answers on here that attempt to cover the basic difference, but the topic is so complicated and with so many subtle exceptions that no short answer could explain all the differences. Further, there are so many interpretations of "shall" that:

The legal reference Words and Phrases dedicates 76 pages to summarizing hundreds of lawsuits that centered around the meaning of the word shall.
From Wikipedia article, original source plainlanguage.gov

And that NASA, the US Department of Defense, the International Organization for Standardization and various others have specifically given their own definition of the word.
Shall and will: Legal and technical use

However my specific question should be easier to handle, as it particularly asks about one use in Psalm 23 from the Bible. In the King James Version it says:

The Lord is my shepherd; I shall not want.

In the Contemporary English Version it's written as:

I will never be in need.

And in others, such as the New International Version it isn't even in the future tense:

The Lord is my shepherd, I lack nothing

I take the "I shall not want" to mean "I will not be lacking".

However a few lines lower in the same psalm it says:

Yea, though I walk through the valley of the shadow of death, I will fear no evil

Since to me the two lines seem to be saying "I will not be lacking" and "I will fear no evil", I'm curious as to why "shall" is used in the earlier verse, while "will" is used in the later verse given that to me they both seem to be an expression of the future (I don't see a difference).

I've found one rule which seems common:

There is nonetheless a traditional rule of prescriptive grammar governing the use of shall and will. According to this rule, when expressing futurity and nothing more, the auxiliary shall is to be used with first person subjects (I and we), and will is to be used in other instances.
Uses of shall and will in expressing futurity

However the subject in both verses is "I", so this rule doesn't apply.

The only thing I can think of is that the translators back in about 1604 - 1611 had intended something different from just expressing simple "futurity" in the earlier verse, maybe something akin to:

The Lord is my shepherd; [it is expected I will not be lacking].

However it's strange I can't find a translation to this effect, the other translation I quoted above says:

I will never be in need.

Is it because one is about desiring and the other is about fearing evil?

Maybe this question might be suited to a Bible site, but I believe the use of "shall" and "will" is really an English question.

If anyone has an idea of why the difference in the two verses, I'd appreciate any information.

Have you looked at the tense and mode cho­sen for these vers­es in oth­er trans­la­tions that use lan­guages with a more clear­ly de­lin­eat­ed tense and mode in­flec­tion­al sys­tem than English has?

Here are two.

Latin

The Latin Vul­gate has for the first verse’s “The LORD is my shep­herd, I shall not want”:

Do­mi­nus re­git me, et ni­hil mi­hi de­er­it.

where de­er­it is the 3rd sin­gu­lar fu­ture ac­tive in­dica­tive of de­sum, mean­ing to be want­ing or lack­ing (to some­one), and then for the fourth verse’s “I will fear no evil”:

non ti­me­bo ma­la

where ti­me­bo is the first-per­son sin­gu­lar fu­ture ac­tive in­dica­tive in­flec­tion of ti­me­re, mean­ing to fear.

Spanish

The Span­ish Rei­na-Va­le­ra An­ti­gua with its KJV-equiv­a­lent old-timey lan­guage has for the first verse:

JE­HO­VA es mi pas­tor; na­da me fal­ta­rá.

where again fal­ta­rá is the third-per­son sin­gu­lar fu­ture in­dica­tive in­flec­tion of fal­tar, mean­ing to be lack­ing (to some­one), and for the fourth verse:

No te­me­ré mal al­gu­no

where again te­me­ré is the first-per­son sin­gu­lar fu­ture in­dica­tive in­flec­tion of te­mer, mean­ing to fear.

Conclusion

So both the Latin and the Span­ish em­ploy the same sim­ple fu­ture in­dica­tives, with­out any signs of the sort of de­on­tic/epis­temic shad­ing that can in the­o­ry oc­cur with the English modals will and shall.

Be­cause those oth­er two trans­la­tions make no dis­tinc­tion there, I would ad­vise against read­ing too much in­to the KJV trans­la­tors’ par­tic­u­lar choic­es of will/shall in these two par­tic­u­lar vers­es.

In order to analyse a translation we have to look at the original it was translated from, although other translations of the same original may give additional insight. But that is where the problems begin:

The original is in Hebrew but I was not sure that was what the KJV was translated from so I looked it up. The answer is so complicated that I can only suggest you read the Wikipedia article, or, at the very least the sections on Considerations for a new version and Literary attributes. The most significant passage is

Old Testament

For their Old Testament, the translators used a text originating in the editions of the Hebrew Rabbinic Bible by Daniel Bomberg (1524/5),[132] but adjusted this to conform to the Greek LXX or Latin Vulgate in passages to which Christian tradition had attached a Christological interpretation.[133] For example, the Septuagint reading "They pierced my hands and my feet" was used in Psalm 22:16 (vs. the Masoretes' reading of the Hebrew "like lions my hands and feet"[134]). Otherwise, however, the Authorized Version is closer to the Hebrew tradition than any previous English translation – especially in making use of the rabbinic commentaries, such as Kimhi, in elucidating obscure passages in the Masoretic Text;[135] earlier versions had been more likely to adopt LXX or Vulgate readings in such places. Following the practice of the Geneva Bible, the books of 1 Esdras and 2 Esdras in the medieval Vulgate Old Testament were renamed 'Ezra' and 'Nehemiah'; 3 Esdras and 4 Esdras in the Apocrypha being renamed '1 Esdras' and '2 Esdras'.

The net result is that there was a range of considerations, but that the main one, at least in passages that were not politically or theologically contentious, was the Hebrew.

So we now turn to the Hebrew text

מִזְמֹ֥ור לְדָוִ֑ד יְהוָ֥ה רֹ֝עִ֗י לֹ֣א אֶחְסָֽר׃

גַּ֤ם כִּֽי־אֵלֵ֨ךְ בְּגֵ֪יא צַלְמָ֡וֶת לֹא־אִ֘ירָ֤א רָ֗ע כִּי־אַתָּ֥ה עִמָּדִ֑י שִׁבְטְךָ֥ וּ֝מִשְׁעַנְתֶּ֗ךָ הֵ֣מָּה יְנַֽחֲמֻֽנִי׃

(Sorry I haven't figured out how to right justify that - can anybody help!)

Now I have a problem because I can't read it, so I turn to

Interlinear v1 and

Interlinear v4

where I learn that the two relevant verbs

2637 [e] ’eḥ·sār. אֶחְסָֽר׃ I shall want V‑Qal‑Imperf‑1cs

3372 [e] ’î·rā אִ֘ירָ֤א I will fear V‑Qal‑Imperf‑1cs

are parsed identically. Although I do not fully understand what is going on, Hebrew tends not to mark tense (as in present, future, etc.) explicitly, but they are both marked as "Imperf" which I think means a continuous sense. But the net result is clear - there is nothing in the original to say that the tenses differ between the two clauses.

So if there is no semantic difference, what is the difference? There are two issues that might affect the translation beyond the semantics. Firstly, this translation, more than most others, aimed for English that sounded good. Secondly, this book in the Bible is meant to be poetry. Some translations (known as Metrical Psalms) aim specifically for a fixed metre, so they can be fitted to a tune. This translation is not one of those, but they would, nevertheless, have taken the natural rhythm of English into account so as provide a text that sounded good when read out in a church. English has a stress on approximately every second syllable and if I try to identify the metre in this passage I get:

{a PSALM of DAVID.} the LORD is my SHEPherd; i SHALL not WANT.

he MAKeth ME to lie DOWN in green PASTures: he LEADeth me beSIDE the STILL WATers.

he resTOReth my SOUL: he LEADeth me in the PATHS of RIGHTeousness for his NAME'S SAKE.

YEA, though i WALK through the VALLey of the SHADow of DEATH, i will FEAR no EVIL: for THOU art WITH me; thy ROD and thy STAFF they COMfort me.

So the first verb is stressed and the second is not. Of course you do not have to stress it this way, but it sounds best to me and so it could have done to the translators. I think SHALL sounds best in stressed position and will in unstressed position. In fact, based on my subjective experience of the KJV, I don't think WILL was ever stressed.

So, in conclusion, I think the difference was that it sounded best with the first verb stressed and not the second, and that they used SHALL when they wanted stress and will when they didn't.

The rule that shall goes with I and will goes with you, he, she, it, and they is no longer used in current-day English, but it came after the King James Bible.

You can see that the rules changed between the 17th century and the 19th century by comparing the frequency of I will and I shall in different works. In Middlemarch, by George Eliot, I will is used 114 times and I shall is used 216 times, nearly twice as many. In the King James Bible, I will is used 2042 times and I shall is only used 196. For contrast, in Middlemarch, it will is used 52 times, and it shall is used once. However, in the KJV, it will is only used 19 times, while it shall is used 682 times.

I don't actually know the full rules that Shakespeare and the King James Bible used for shall and will, but one aspect is that shall was more used just for simple futurity, while will was for volition on the part of the subject. Since when you use I will, volition is usually involved, one would expect I will to be more common. And since things do not have volition, you would expect it shall to be more common.

I believe the OP's example reflects this:

The Lord is my shepherd; I shall not want.

Here, whether I want or not is beyond my control, so I use shall.

Yea, though I walk through the valley of the shadow of death, I will fear no evil.

Here, I have some control over whether I will fear evil or not, so I use will.

The actual rules were probably considerably more complicated than this (although maybe the people who translated the Bible broke them when they thought it sounded better); will is occasionally used with it for things that have no volition (As in Matthew 16:13 "And in the morning, It will be foul weather to day: for the sky is red and lowring.") And sometimes I shall and I will are used in ways that seem to contradict this rule.