Ruth Ellis was obsessively in love with, and emotionally addicted to the love-and-abuse behaviours of the very handsome racing driver, David Blakely. (Her abuse addiction was formed by the childhood sexual abuse from her father.) It is said that Ruth, as a very damaged person, could be equally cruel to David. They were both deeply dysfunctional.
Ruth killed her lover David due to his ultimate rejection of her. He promised to marry her, then without telling her she heard from someone else that he was planning to marry another woman. She did not want anyone else to have him, if she couldn't.
In a way, Ruth killed David in cold blood. It would have been a crime of passion if she had shot him with a gun already in her possession, and in the heat of a terrible row - or very shortly afterwards while still in a heightened state of distress.
However, her aquisition of the gun was premeditated. She appeared shockingly cold and calm at the later date when she deliberately riddled David Blakely with 5 bullets. (There were several witnesses.) Apparently, she had planned the sixth bullet for herself, but as she held the gun to her own head - the barrel jammed. She most definitely wanted to die with him. She did not want anyone else to have David.
As Ruth was the generation brought up with much stronger Christian values than later generations, she totally accepted the Biblical Old and New Testament "An Eye for an Eye" theory. Furthermore, she may have also powerfully believed that she would be re-united with David in the afterlife - which is why she was genuinely happy to die with him. In her twisted logic, this was a way she could have David entirely to herself - and for eternity!!! In these particular circumstances, if there hadn't been a death penalty, I think there is a strong chance Ruth would have found another way to kill herself, so she could join her beloved David in the "afterlife".
It is absolutely right the death penalty was abolished shortly after that. There have been many cruelly abused women over the years who have ended up killing their partner either in self defence, or from a genuine crime of passion.