To put it simple, zeref loves natsu because he is his brother & tried many ways to revive him but people won't come back to life & the only way for him to make natsu exist in this world was making him into demon.
Zeref did loved mavis or maybe still love her until now but he was mortified because of him, Mavis died when she was supposed to be immortal like him.
Ankseram curse can change people's mind like having split or two personalities. When his heart couldn't no longer bear the pressure, his dark side took over and will wish for nothing but annahilation.
I don't like it at all, that Natsu is a demon. It makes me think that maybe he's strong just because of that. And the thought that he's not human...
Hm.. But Mavis isn't dead, right?! She's just in a frozen state, she never really died.
I hope Zeref will turn back to being a good guy again. I feel so sorry for him and Mavis...
I don't get it. Didn't Zereff create Natsu as a way to be killed by him? And didn't he kind of love Marvis? So why does he want to kill them now?
And didn't he say once that he wouldn't do anything in this era? What made him change his mind?