Being a "lefty", I think whatever goes around comes around. If it is ever determined who off him, they will end up worse off. Is that justice?
According to Wiki, "Full Sutton is operated by Her Majesty's Prison Service and held some 596 inmates, as of December 2016. The prison's primary function is to hold, in conditions of high security, some of the most difficult and dangerous criminals in the country."
So more than likely a lifer killed him. Doubt he cares what happens to him since it won't matter.