Ultear brain washed and forced Erza best friend/love interest into enslaving and killing several people, gets a pass after being sorry. Gajeel beat the crap out of two of their members, get recruited spite of his reluctance, Erza goes out of her way to redeem Minerva in spite of what she's done.
Zeref the man who admitted to have gotten himself cursed to bring back his little brother, cursed with the most horrifying and painful curse imaginable, trained FT with no benefit to himself and out of kindness done for him. His own breaking point came after opening his heart and killing the one he loves..... this person deserved no quarter or empathy.
Look I can get the lack of diplomacy. What bugged me about it all, was the way Natsu treated zeref in their fight. What he was saying was meant to sound as though zeref is trying hard enough, but the way it came sounded like natsu was just targeting zeref thinking the world was rejecting him, when the curse requires him to not care for the world so everywhere he goes doesn't turn into a desert. This was in spite of the fact that Natsu knew zeref for who he was more then anybody. The complete lack of empathy was out of character for natsu and FT as a whole.