You have to try and smile, Zeref

Cute picture, but I simply cannot really like this this pairing anymore, as Zeref is fully able to sacrifice Mavis just for the selfish goal of doing away his pain. Yes, he is tragic, but he wants now to erase the whole timeline, meaning to achieve his own personal happiness, he will commit the greatest act of murder imaginable (omnicide). And yes, it's described he will kill/erase everyone this way, just to create a world were he is not cursed, and he told he doesn't care about it anymore (and yes killing everyone including Natsu and Mavis and he in fact would kill them even before that).

Zeref saddly pretty much crossed the Moral Event Horizon as this point, and I think a happy ending for him, would be a cop-out by Mashima, because really, I think he simply doesn't really deserve it, as he shown he is wlling to kill every person he loves, out of share selfishness...