Natsu is my favorite character in fairy tail, but from the moment Zeref appeared I felt there was going to be something special about him. When I found out who he was I was surprised. When I had heard of Zeref I imagined this scarier looking bigger person. And then to add onto it he didn't appear to be a bad person, just someone who was killing because he couldn't control the power. After discovering Zerefs past I really admire Zeref now. And despite the fact that he is forced to play the bad guy, I don't see him that way. He has now become my second most favorite character moving Gildarts to third place.
Zeref was studying dark magic to bring back his brother who had died of a sickness a.k.a. Natsu. This for me showed that he wasn't a bad person, he just wanted his brother back (although I did briefly question why it was just Natsu and not their parents too). But because of the magic he was studying he was cursed with a life of immortality and not just that but a life where the more he loves things the more they are destroyed. So in the end he is forced to hate everything so that it won't be destroyed.
And then just when he finds someone who is forced to suffer the same fate as him, falls in love with her, and thinks that he's finally not alone, he ends up being the one that causes her death driving him even more mad than he was. It was a horrible fate. and so despite the fact that he is the bad guy, I still admire him.