Hmmm...
I think that sometimes, the "evil character" doesn't do things 'cos of right or wrong. Sometimes, he knows it's wrong but he's overcome with so much grief, suffering and hatred that he just doesn't care anymore or he wants vengeance on the rest of the world.
It's like "you(the minions, etc.) can do whatever you want, just leave me alone" or even "It's those bastards again. I don't care whether you're friends of those who killed my girl but you will perish! Everything and anyone who shares the same ideals as the forces of _____ must die."
Anyways, why apply "evil" to a character? Due to the moral and social codes of a time period, your deeds might be considered "evil": like the execution of "witches" and "sorcerers" is approved by the religious orders, to release them is a heinous deed. Or "to take pity on and shelter the lower castes" is a blasphemy which results in hanging or torture.
Whereas "good" could mean "slaughtering the men, women and children of your enemies" or making slaves out of the enemies like the Raanars did.
And I've always liked a character redeeming himself but that would mean you and the other characters must be able to help him in times of need. I've never believed in leaving someone to the wolves: you either finish the job of fixing him up or don't do it. It's just like a surgeon vanishing before the operation has ended.
If a character breaks 'cos he's facing reality, then there must also be ways for him to recover. Otherwise, that could have rather unpleasant effects: like him not recognising anyone and wildly attacking random people or even trying to harm himself repeatedly.
Having known someone who actually broke, I'd say it's a rather unpleasant experience. To lose almost all your emotions and become a shell of a person and sometimes fall into a daze, to fight a battle between hatred and empathy, despair and kindness can turn you into a living dead. To make things worse, if you're overcome with hatred and the instinct to harm and hurt others, you might actually also try to destroy your own feelings and twist them for your own purposes.
