When I started One Piece, I remember thinking that Sanji’s backstory was the most tragic along with Nami’s and Robin’s. All involve death, but these three explore the depths of human remorse much better. Robin turns herself into a ruthless criminal to survive, Nami into a thief, and we can see her greed for money perfectly explained, and Sanji, after losing his whole crew - back then we could think the Orbit was kind of his family -, is ready to murder to survive.
They are the best because they explain how some traumatic events show us how even a innocent child can easily turn into a monster, and how it is for those who survived them to go back to a normal life and to have normal healthy relationships with others, how to be able to trust again.
As I said, I thought it was really painful, but out of the three, Sanji’s was probably the less traumatic.
Nami’s overcoming of her past was shown carefully in Arlong Park, very early in the manga. Since then, she’s gone through relatively small changes in the trust she has in ner nakama.
Robin’s started back then in Enies Lobby, but was only completed just recently, when questioned by Inuarashi about her safety, she assured him she trusts her life completely in the hands of her comrades.
For what concerns Sanji, we deemed it was completed with Zeff’s meeting, but just because we thought that was all there could be to it. Now, unlike any other character we could suspect of except probably Brook. Sanji’s trembling told us it wasn’t overcome at all.
And how strange is it at chapter 838? I think Sanji’s definitely a special characters to Oda.