Just like Sarmad and Razh said, I also believe in such a theory concerning Roger's capture. There has to be a reason; I think the WG must have threatened him with something very serious. In the end he must have had no other choice than his execution...
Speculation: maybe it had something to do with his family, friends, wife even? Do you guys think Gol D Roger was married? Or are all great men too 'wild' to have a wife and get settled? xD
The wife of the Pirate King, lol, sounds funny! ^^
Anyways, there could be a lot of reasons for his capture/turning himself in, but I'm pretty sure it was because he had no other choice and not because he was overpowered by someone else.
Concerning Rayleigh: I do hope he's a good guy, he could really be helpful to the SH when it comes to answering some questions (like IgnorantSage said). I would love to hear Nami ask him if One Piece really is the biggest treasure you can imagine and see her eyes turn all belli-like.
