He's the least of the problems with the ranks of the Ten Wizard Saints. { Nope. Not morality. Politics, however, seems to be the only basis for it, at least in Zeref's opinion. }

Don't underestimate his bloodlust. I said once before that I found his motivations depressingly shallow. Destruction for destruction's sake. The only reason he hadn't made his move sooner is that, were he to succeed, he'd spend the rest of his life even more bored than he already is.

But as we've discovered, it doesn't take much to get him to attempt to at least partially whet his appetite for carnage.