I think the death penalty should be kept - or reinstated in the case of the UK - so that the worst criminals are eliminated from society.
Their is a myth going around that prison sentences are to rehabilitate the criminal. This is false - sentences are there as a punishment for serious crime.
Some people who commit very serious crimes are beyond rehabilitation, and deserve to be put to death (e.g. Peter Sutcliffe aka the Yorkshire Ripper).
Of course, having the death penalty on the books can help a criminal see sense and confess in exchange for a life sentence. This is another reason to keep it/reinstate it.