User blog comment:Safaia Mukade/my questions/@comment-6152051-20131019194538/@comment-2186265-20131021092036

I believe what Zeref himself has said was that a long time ago he started to care about life and because of that he lost the ability to control his Death Magic, thus causing it to randomly kill things. Thus when he came face to face with Grimore Heart he no longer cared for human life and so he could fully use it then.