1) Philosophically - depending which belief system there is always a point you can return from (mercy)
2) Point of no return for villians, this is an oxymoron for me. Because it implies the moment where the villian ceases to be one.
3) I guess for me, it be villians taking pleasure in the psychological breakdown of their targets. (This incorporates rape as a tool, machinations of depravity, forcing characters to utter despair)
To expand on that, Villians who are utter trash, that subject other characters to become their tools for their desires. Its like the corruption on innocent youth, molding the young into tools for destruction or for their profit.
These are villians, I can't forgive. for they do not offer the decency of equality, but maintain rigid authority of those weaker and vulnerable to them.
4) While certain belief systems, teach one to be magnanimous (merciful), it is hard for me to be merciful to one that in the end, will never repent (and villians never will)