The question is whether you should imprison, or kill that person. They are two very different things.
The difference is between murder and manslaughter. Should Sally Challen have been executed? She was convicted and subsequently acquitted of murder but the fact she killed her husband was never in question.
https://www.bbc.co.uk/news/uk-england-47407204
No, she should have been convicted of manslaughter not murder. I'm not saying people should be executed instantly after conviction, but for the most heinous crimes I believe it should be an option.