Zeref wasn't evil to begin with, he's cursed remember? If he values life, he can't control his curse which as a result will sporadically 'combust' and kill everything in the vicinity. The only way for him to control that curse, is to stop valuing life. He's not so much 'evil' by choice as he was forced to become it. This is also what he was referring to in the latest chapter: it's his fate, it's not something he can do anything about it.