1-Killer’s helmet has been noted to bear resemblance to the French musician Guy-Manuel de Homem-Christo’s helmet from Daft Punk. Daft Punk was made famous in Japan by their animated film Interstella 5555: The 5tory of the 5ecret 5tar 5ystem, during which time the duo were wearing outfits designed to make them look like robots.
2-Killer and Zoro share a number of similarities, including their inclination towards killing/violence, status as the only non-captain Supernovas, lack of an apparent position in their crew, and a fighting style that includes blades. Also, both are the only members of the Worst Generation without powers of a Devil Fruit (although Killer has the weaknesses of a user by having eaten a defective SMILE).
3-According to Kid, Killer hated his own laugh so much that he beat up anyone who made fun of it. He eventually stopped laughing at all and started to wear a mask to hide his face.
4-The water torture that Killer and Kid were subjected to is a version of the old Japanese torture and execution method known as tsurushi or “reverse hanging”.
5-Oda said he picked the name “Killer” out of sheer laziness, thus the name holds little to no meaning. It means that he is the only Supernova introduced in the Sabaody Archipelago Arc whose name is not linked to a real life pirate or privateer and the only one of the group with no other meaning at all.
6-As stated in SBS, Killer eats with his mask on, and prefers food he can consume through the holes of his mask (stick-shaped or foods such as ramen). His meals have pasta as main every day and his favorite is Aglio e Olio. He also drinks alcohol through a straw.
7-Killer’s birthday, February 2nd, comes from his epithet “Massacre Soldier” (殺戮武人 Satsuriku Bujin) since 2-2 can be derived from bu-ji. shares the same birthday date with Vivi.
8-If One Piece was set in the real world, then Killer would be from Scotland.