If you are reading this, then you must have read my first predication, if you haven't, then do... Now that I's done, read on.
In my predication, Bartolomeo is a young strapping lad who just happens to be a pirate... He is also a mass killer, psychopath, and just plain a horrible person. The thing that makes him different from every other character is that he loves Luffy (In a way a student loves his teacher that he respects so much he wants to be aside him in that way, not the way you thought!) Now you're just thinking, Nobody, he is a fictional person you made, and if you have that problem, then you can leave but this is to explain why he is like this, and maybe how he MIGHT BE (Don't quote me on this or I'll block you from this blog if I'm proven wrong).
He is a teenager with not so many screws on his head and no one tried to straighten them, so you can see why he acts a little different, but he thinks Luffy is like him. He does not love Luffy because he attacked Enies lobby, A world noble, Impel down, and Maineford twice, but because he DID them. He thinks that Luffy is just like him, a crazy psychopath who wants to kill everyone and everything that messes with him. He does not like Kidd, even though Kidd is more of him, because he did not do any of these things. Bartolomeo wants to be a straw hat, because he thinks they are just like him, but really, they aren't even a bit like him at all. They have people who love them, cheer them on, and will pick them up when they fall. He has none of those things, he was abounded, kicked around, and laughed at. All he had was newspapers, and every major headline was of someone his own age attacking everything that pissed him off (As they say most likely). So why not be him, or try, or better yet, join him. He became a pirate only to be with Luffy, someone who he thinks will be like him, help him, and kill with him. To all points, Bartolomeo has no crew, only making it this far by invading other ships and killing the crew when he was done and do it again.
To sum him up, he is not some person I think will join, but someone who try's, and will never be even close, because he thinks Luffy kills his crew when they make him angry, kills everyone who angers him, and has no respect for anything. The only reason why he joined the coliseum was to give Luffy his brothers devil fruit, to which, he thinks Luffy does not cherish aces memory, but of aces power only clinching why he will never join.