I realise there have been cases in the past where the wrong person has been sentenced to death. And I agree that it does indeed cost a lot to keep a person in prison for donkey's years.
I always ask myself when pondering these things, "Would I be prepared to press the lever and kill off a murderer?". My answer would be "No!", so I suppose I shouldn't expect anyone else to do it either.
However, self-defence is a right. If a person entered my house and tried to kill my husband, I'd surely bash them over the head with my heaviest frying pan and do them in!
Assisted dying is a hard one to decide. A person in terrible agony with a terminal illness can be helped to end their unendurable life.
While prison visiting (eight UK prisons, some Category A) I learned that most of the inmates did not change, reform or become less dangerous, so were fairly likely to re-offend. A 'life sentence' for murder should mean 'life' perhaps.
I'm finding this quite a difficult subject on which to form an opinion.