Idiom or proverb request for describing or criticising a situation in which the lowliest dare mock a mightiest who has become old

Solution 1:

"How the mighty have fallen!" Is fairly close in applicability and intent. It's from the Bible (2 Samuel 1:27). See here for some discussion.

Solution 2:

A reference to "Ozymandias" or "Look on my Works, ye Mighty, and despair!" mocks someone who used to be great, but through the passage of time has been eroded into oblivion. The quotations are from a poem by Percy Shelley:

I met a traveller from an antique land,
Who said—“Two vast and trunkless legs of stone
Stand in the desert. . . . Near them, on the sand,
Half sunk a shattered visage lies, whose frown,
And wrinkled lip, and sneer of cold command,
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things,
The hand that mocked them, and the heart that fed;
And on the pedestal, these words appear:
My name is Ozymandias, King of Kings;
Look on my Works, ye Mighty, and despair!
Nothing beside remains. Round the decay
Of that colossal Wreck, boundless and bare
The lone and level sands stretch far away.”

Solution 3:

A very general expression of the fact that many people act braver in safety than in danger appears in Charles Pidgin, Theodosia, the First Gentlewoman of Her Time (1907):

We are all brave when the danger is past.

Jon Stone, The Routledge Book of World Proverbs (2006) contains a similar saying which it identifies as an Italian proverb:

All are brave when the enemy flees.

Stone also has a rather ironical proverb related to the changed attitude people have toward a powerful person who is no longer powerful, this one from Ireland:

The Irish forgive their great men when they are safely buried.

None of these examples is entirely on point with regard to mockery by the weak toward the formerly strong. Perhaps the closest match on that point is (again from Stone's book) this Italian proverb:

When the lion is dead the hares jump upon its carcass.

But whereas I've heard versions of the first three expressions spoken in English, I've never heard anything like this last one.