So I'm like 99% sure that Future Rogue is Acnologia in the making. That would explain his 180 turn in personality. And I'm pretty sure he fits one of the said requirements of 'bathing in too much dragon blood'. Maybe he wanted more dragon blood to 'bath in' because he's killed all the ones from his time. That could mean that the evil aura he gives off could be from his transformation from insane dragon slayer to insane dragon. It could also be the stench of having actually bathed in dragon blood. Maybe he just needs a shower.
Maybe you guys have already agreed on or crossed this theory out. Forgive me. You may punch me five times for my negligence.
Also, if Lucy were to be helped by Zeref, ---to travel through time/save the future/whatever tragedy she'll need help preventing/(tie her shoe?)--- I think the sacrifice would be more than just her arm. In the draft doesn't Zeref say something about her singing a song because she'll never see Natsu again? Then again, I don't think his drafts are very reliable...
You may punch me another five times for my use of unreliable sources.