User blog comment:Omega natsu2/Chapter 387: Tartarus Arc, Part 4: Father and Son, Predictions./@comment-121.54.54.35-20140602022846/@comment-7418318-20140602095938

I like that theory. It also opens up some other interesting possibilities.

Your post made me think Silver could be a kind of executioner for Tartaros who hunts down those Demons who refused to follow E.N.D.

Zeref said that E.N.D. created Tartaros on its own. Since it wasn't created by their creator Zeref, there might be some Demons who didn't want to follow this "upstart" and so E.N.D. (whether directly or through Mard Geer) ordered Silver to hunt those "renegades" down and either force them to join or annihilate them.

Failing in his task (whether it was appointed or chosen by himself) because a Human, who the Demons consider "less than insects", did Deliora in before he could must have been quite the blow to Silver even by itself. And any kind of romance would only strengthen his misgivings for the ones responsible for Deliora's demise. But with Ul being out of his reach, all his hatred concentrates solely on Gray.

And if Silver wanted to go hunt down Gray, but couldn't because of his duties or orders not to interfere with the Humans yet, that hatred would only have grown over the years.