User blog comment:Omega natsu2/Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Predictions./@comment-123.2.147.192-20140212065209/@comment-24066070-20140212100026

When confronting Precht, Zeref did say his body "received a curse" and said curse is the thing which causes the death aura to shoot out of his body.

So perhaps Zeref has his own curse or he always used curses and just developped magic as a toned-down curse for humans to use.