I doubt its Iceman, I read somewhere on top of him being Omega level, hes that much in control of his powers hes virtually unkillable, his molecules just realign again. It has to be someone with shock value but has nothing left of value to add to the X-Men anymore OR the complete opposite, someone that would have saved mutantkind but was killed/died and prevented it being done the easy way.
Beast is a logical choice because he can be both, PLUS he did the unthinkable and explored the dark side of genetics. The other thing is he has a replacement, Dark Beast is a lunatic, but will sit well with the X-Men's hardline stance if it continues.
I'm going to throw a "wildcard" out there and suggest its Gambit. If you look at Rogue being slightly insane I'd say she would be the one that kills him, for the sake of the X-Men's survival, but either unaware or forced to. We know how indiscriminate Marvel was with killing people in Civil War (they killed freak'n Cap for Christ's sake) and Gambit hasn't really been in a core book since God knows.