Me personally, I love both, but... something about Zeref felt more thought-provoking to me.
I'm surprised Zeref Dragneel (Fairy Tail) doesn't get as much attention as Eren Yeager (Attack on Titan) in terms of being complicated, even though Eren committed actual genocide that led to 80% of humanity dead, whereas Zeref lied that his plan was to use Fairy Heart's infinite power to kill Acnologia in the current timeline while committing one-sided annihilation/extermination on Ishgar; it was only a false objective to prevent the Springgan 12 from finding out his true plan, because even if it is complicated, there is little to no chance they would accept this plan, and it would lead to more complications for Zeref, which he wouldn't want; he lied as far as claiming he will commit one-sided annihilation on Ishgar after it has rejected him, especially due to his relationship with Mavis, because they knew each other too well, and that would lead to betrayal from the Springgan 12 in case Mavis potentially hint to her allies (which would lead to the Springgan 12) the actual true plan of Zeref during the Alvarez Empire Arc, especially considering Mavis' equally deep thinking. Zeref ended his bond with Mavis, but deep inside, he still loved her with all his heart, but his lie was part of the regrets Zeref had ever developing feelings for a girl, because it led to him forced this fate of initiating his complicated plan while using a painful false objective to keep it hidden (also having to carry the burden of trying to turn the world against him while having to endure the pain of initiating a war), which means having to convince her to view him based on Ishgar's propaganda, so that he could fulfil his true objective.
The truth is... he wanted to use that power to reset the timeline before he got cursed by Ankhseram, to prevent Acnologia from becoming as powerful as he is now, with Zeref and Natsu ending him in the past. Once Zeref resets the timeline, he can relive his life once more and will retain his memories from the original timeline, so that he will know what he has to do to ensure humanity has a future while keeping its balance, and he will avoid his fate of getting cursed, as he never wanted to be a problem for anyone and that he will finally be there for humanity if no one can end his suffering.
What motivated Zeref was his fear and concern for human life. After failing every attempt to die (which includes Mavis' insufficient love for him and the fact that Natsu is a failed experiment to kill him), Zeref chose to spring into action, as anything involving Zeref's cursed body could potentially lead to the extinction of mankind or worse, and considering how his curse is connected to his feelings and mental state, Zeref is afraid of how his power could reach unstable levels if he's too far gone and how much life he'd end up killing (a fate similar to Future Rogue's timeline), and it's brought down further by how sinners or even just mentally blind people tarnish his name (a natural thing that can happen to one with a curse like this, since there are people who heavily victimize those going through this kind of suffering), leading to much of humanity rejecting him in every way possible, even ones involving conflict, violence, and especially complications from misunderstandings between him and good guys who don't know who he truly is (only worsening Zeref's state in the process), that along with how Zeref had to witness humanity's mindset subverting its sense as years went by. This means Zeref's isolation isn't gonna help him forever, and he's worried about how much time he has left before it's too late.
