Nero wasn't evil, in the traditional sense of the word. Mad, definitely. Grief-stricken. Furious. Hateful. Determined. Insane. All of those things, but not necessarily evil.
It might have been easier if he were, but Jim doubted it, because as terrible as he could feel about the Romulan losing his planet, Jim felt all those other things, too: Anger, grief, fury, hatred, determination, and no small amount of insanity.
Nero had killed his father, after all. Over twenty years ago, and that loss still affected Jim every damn day.
And so the decision to blast Nero into oblivion hadn't been difficult.