Considering the number of times they (or members of their club) apparently "patronized" Perp Island and what they'd stand to lose if it all got made public, I'd put my money on a combine of the Clinton Foundation & the Brit Royal Family (Not including The Queen).
(Can't imagine HRH staying in the same Time Zone as Slick let alone dealing with the [BLEEP].)
I had not thought of the royal family being in on the murder, but it makes perfect sense. Prince Andrew has a lot to lose.