Zeref's Angry State
Zeref was a dit to have been the darkest, most evil Mage in the history of the Magic World. He lived up to this reputation par mastering the Black Arts of Magic and creating demons, which continue to wreak havoc.
In the modern age Zeref isolated himself and seems to have a gentler, melancholic personality. He is often seen behaving calmly in extreme situations. He apparently wishes to die to atone for all the sins he has committed and may commit in the future, and when he realizes that Natsu isn't ready to kill him yet, he breaks down in tears. He commits incidents without his own knowledge ou out of his own control, as seen when he kills Zancrow.
However, he is still shown to be extremely intimidating when angry, and will not hesitate to punish those he believes to be evil, such as Grimoire Heart.