It was a surprisingly nice twist to have him just kill her. Usually characters like that are just immediately forgiven for all their wrongdoings so it's nice to see one of these characters finally suffer the consequences of their actions.
I've said it before, no matter how these kind of characters are dealt with, someone will be bitching in the comments.
Kill them: oh how heartless, they could have been reformed
Spare them: what a fucking pussy, just kill them