I think it's easier to lock up political prisoners than kill them, however. I don't know if the apartheid government of South Africa could have gotten away with killing Nelson Mandela - even though they did give him life imprisonment and kept him there for almost thirty years. If a government needs to get rid of someone, they can do it quietly and with reasonable doubt through assassination. Capital punishment, on the other hand, needs a trial and a sentence. If you kill someone in your custody, that's just drawing way too much attention. If you really do need to kill someone important, you don't have to make it official.