Moral Event Horizon
In TV Tropes terms, "The Moral Event Horizon" refers to when a villain does something so terrible that they have crossed the point of no return. They have established themselves as irredeemably evil. This term is named after the part of the Black Hole that once crossed there is no escape. Anything that goes past the Event Horizon is lost forever. So its an appropriate label. Note that while a crossing of this can also establish a villain as pure evil, it needn't be so. They've just established themselves as irredeemable. Pure evil and irredeemably evil, while usually going hand in hand, don't always have to.
On that note, this is a list of various fictional characters and their Moral Event Horizon crossings. I may add to this list later.