(WARNING! Spoilers for One Piece Chapter 814)
I always wondered why Sanji was so ready to kill someone, he’s a kid, and most kids wouldn’t just pick up a weapon and declare they are going to kill you. (Sanji genuinely planned to try to kill the pirates and Zeff, he’s not bluffing)
I assumed it was due to a traumatising event that happened before he got to East Blue, but know we know that Sanji’s family are assassins and this makes so much more sense. Of course Sanji’s first instinct it to kill, he’s been raised as an assassin. If The Vinsmokes being assassins was Oda’s plan from the start, then he did an amazing job at foreshowing this in Sanji’s flashback. Sanji’s habit of sneaking off in many arcs (Little garden, Water7, Enies Lobby ect) also ties in with his family’s occupation. Assassins are good at sneaking around undetected, and Sanji has done this throughout the series.