Well, let’s begin.
-Sanji never hits a woman, right?
-There was one instance when he actually tried to. That was when he fought Kalifa. Sanji tried to finish the fight by pinning her down with his leg, but she escaped. Kalifa then called Sanji a coward, and he lost it. He yelled at her, telling her to shut up, and then he tried to kick her. She managed to dodge the kick, but if she wasn’t so quick, the kick would’ve definitely connected.
-So Kalifa calling him a coward made him so angry that he abandoned his chivalry and attempted to kick a woman? This leads me to believe that “coward” triggered a traumatic memory, which is why he lost it. Perhaps his father used to beat up his mother, and his mother would retaliate by calling him a coward. Sanji hates his father for that, and he nearly lost it when he thought about him. This is a bit unlikely, since hitting Kalifa would only make Sanji act like his father even more.
-So perhaps his father used to beat up his mother, and Sanji never helped. One day, his father went too far, and killed his mother. In her last dying breath, she managed to call Sanji a coward for not helping her. Kalifa calling him a coward made him remember how he didn’t help his mother when she needed him, and he didn’t want the same thing to happen to Robin, so he got angry and tried to get the key with force.
-It’s possible that he ran away from home after this incident. He probably snuck onto the Orbit, where he learned how to cook. Maybe his mother was a cook, or maybe she wanted him to become a cook, and Sanji wanted to honor her.
-His father must have been looking for him all this time. When Sanji got his first bounty, his father probably thought it was somebody else (since the picture looked like Duval). When the real picture was revealed after Dressrosa, he must’ve requested him alive so he could successfully join forces with the Charlotte family through Sanji. But how would somebody like Sanji’s father make Sanji wanted alive only, especially when the Germa 66 have been fighting the Marines?
-It’s possible that he was hired by the government to keep an eye on Big Mom, or maybe he was hired to let the government know about underworld events. Maybe he blackmailed them, like Doflamingo did. Either way, the Government is hiding something. Maybe they’re secretly working together. Quid pro quo, eh?
-Who is his father though? I believe we have already seen him. Vinsmoke has the words Vin (wine) and smoke in it. This underworld broker is smoking while drinking wine. Not only that, there are curls under his wine glass, similar to Sanji’s eyebrows. The other guy there might be one of the brothers.
-There was one more instance when Sanji abandoned his chivalry. That was in Jaya. He told Nami he would protect her no matter what was in the forest, yet when a tarantula appeared, he ran away and told Usopp to get rid of it. When a bunch of moths and centipedes appeared, he acted the same way, even though he told Nami he’d protect her no matter what was in the forest. He said he couldn’t stand creepy crawlies, but he wasn’t scared of the giant ladybugs, so it was only the tarantula, moths, and centipedes that scared him. I have a feeling that those 3 have something to do with his past. Maybe those 3 represent his father and 2 older brothers. Since the tarantula appeared first, that’s probably the one that represents his father. Maybe he has a tarantula Devil Fruit. Since he’s an assassin, the powers of a tarantula would suit him well.
-One son might have a moth Devil Fruit, another Devil Fruit that would suit an assassin. Most moths are capable of flight, poison generation, wall crawling camouflage, and enhanced senses, which include night vision.
-Well what about the second brother? I believe he is Killer. Yes, I know that’s a very common theory, but I have my own original reasoning for believing so. Killer’s favorite food is pasta, which is Sanji’s cooking specialty. That’s not all. So the father represents the tarantula, and one son represents the moth. That son could be Killer, but I don’t think so. That would mean that Killer represents the centipede, and believe it or not, he shares many similarities with the centipede. There is a centipede called the Killer Centipede. They share “Killer” in their name. The Killer Centipede has sharp hooks on its body, which it uses for offensive purposes, just like Killer. It’s also blind, and it uses advanced senses to move around. I’m not sure if Killer is blind, but seriously, how does he see with that mask? Observation Haki? In other words, advanced senses?
-Putting Sanji’s father aside, maybe Onigumo has something to do with Sanji’s fear of tarantulas. He does seem to have a spider Devil Fruit rather than a tarantula, but hey, Tsuru was stationed in North Blue, maybe Onigumo was stationed there as well.
-Sanji will probably face off against his father at some point, and if he does, he might use knives. Why? Because fried spider is a delicacy in some cultures, so if his father is in his Zoan form, Sanji might bend the rules.
*Theory by Samurai Rengoku