alright i’m gonna talk about my zosan death pact au a bit because i think i hit upon an interesting angle for heartless!sanji that i haven’t seen people take.
first of all, i don't call him emotionless, because i don’t like the idea that having emotions or empathy constitutes a good person. i personally have low empathy and pretty flat emotions, actually. also, i tried my best to write heartless!sanji in a way where it didn’t suggest DID. i’m also plural, and i’m not a fan of the “evil alter” trope. i don’t think i achieved all of this perfectly, because the source material kind of suggests a lot of this stuff in itself, but i really tried.
so my heartless!sanji is based on a very specific premise: what if sanji was coldly and ruthlessly pragmatic about removing anything he saw as an obstacle to luffy’s journey? this is based on this page from chapter 1031:
sanji is extremely focused on what would be useful to luffy. he specifically brings up his inability to fight women as a weakness that he could get rid of if he didn't let his principles/sentiment stand in the way. it's all centered around luffy, first and foremost, and his place on luffy's crew. so when i was writing heartless!sanji, i didn't make it so that he didn't know who he was. he knew he was sanji, he knew he had certain principles, he just chose to disregard them because he thought that they would hold luffy back. i was very specific that what sanji loses is his kindness, his ability to view the things as existing beyond means to an end (luffy as pirate king.)
and then i was thinking, well, there's one person that sanji doesn't treat with much kindness — himself. in fact, sanji's refusal to see himself as something other than a means to an end is kind of what landed him in this situation in the first place. so i zeroed in on this. heartless!sanji isn't a completely different person — heartless!sanji is what sanji would be like if he treated other people and his principles the way he treats himself.
so the reason why he fights and "kills" zoro is because after zoro challenges him, heartless!sanji sees him as an obstacle to luffy's journey that must be removed (... a liability, if you will.) after he regains his kindness and regrets his actions, he takes the natural next step — he begins viewing himself as the obstacle. means and ends.
i personally am pretty happy with this premise because i think it leaves things more open-ended, and also creates opportunities for interesting character growth. there's more to the way i wrote sanji (and zoro) in the comic, but this is a very big throughline in it that i was happy to have come up with.