A word for making idols from the dead ( Deadolatry ?)

The word you're looking for is necrolatry.

Which term to use depends on the attitude you want to express. The first term I thought of was ancestor worship:

the custom of venerating deceased ancestors who are considered still a part of the family and whose spirits are believed to have the power to intervene in the affairs of the living

(Merriam Webster)

This word is relatively neutral, although it might have a negative connotation to some people (usually a negative sense would arise from an implicit religious assumption that only God is worthy of "worship").

However, Wikipedia's article about this practice is called "Veneration of the dead," and it includes the following section discussing terminology:

Most cultures who practice ancestor veneration do not call it "ancestor worship." In English, the word worship usually refers to the reverent love and devotion accorded a deity (god) or God. However, in other cultures, this act of worship does not confer any belief that the departed ancestors have become some kind of deity. Rather, the act is a way to respect, honor and look after ancestors in their afterlives as well as seek their guidance for their living descendants. In this regard, many cultures and religions have similar practices. Some may visit the graves of their parents or other ancestors, leave flowers and pray to them in order to honor and remember them, while also asking their ancestors to continue to look after them. However, this would not be considered as worshipping them since the term worship shows no such meaning.

In that sense the phrase ancestor veneration may convey a more accurate sense of what practitioners, such as the Chinese and other Buddhist-influenced and Confucian-influenced societies, as well as the African and European cultures see themselves as doing. This is consistent with the meaning of the word veneration in English, that is great respect or reverence caused by the dignity, wisdom, or dedication of a person.

So apparently "ancestor veneration" may be a more PC or neutral-sounding word. You should take this with a grain of salt though, as I'm not sure whether the ideas expressed in the Wikipedia article are drawn from some established field of study like anthropology, or just one person's preferences (most of the citations in this passage are just to general-purpose dictionaries).

"Deadolatry" is not a word that I've ever heard before, and it sounds funny to me. I'd advise against using it. Deadrat's "necrolatry" sounds better. To me, words like this that are based on "idolatry" have a definite negative connotation (which may well be what you want). The Oxford Dictionaries define it as

Worship of or excessive reverence for the dead