To be fair, considering what men have done to women and girls historically, and what still goes on now, globally - the dehumanisation, the powerlessness, the depravity, the objectification, the enabling of each others' behaviour, and the utter apathy over their fellows' misogyny even if they're not 'actively' misogynistic themselves...
Can you really blame women for being satisfied when a man who murdered two girls is killed? Do you really expect a woman to have any empathy for a man like that?
I'd knit at the gallows quite happily over a man who took girls' lives without so much as a thought to their humanity, and fed on their fear.
Sure, the prison system shouldn't allow for a prisoner to get hold of a weapon, if only because they could hurt prison employees, or traumatise them with the violence they do...but it did happen, and rather than a prison employee being hurt, a murderer was killed by a man who will never be released from prison. Compared to what could've happened, it seems like a satisfactory outcome.