Horrible things happen during war. This is a good example of why we shouldn’t take the law into our own hands, no matter how tempting it may seem at the time.
Agree with you on all points, but would also caveat that in the wake of evil that descended upon Europe during the war, some reactions to it have to be understood in context. In "Band OF Brothers" there's a scene set in the immediate post-war Austrian Alps where some of the soldiers go to a small mountain cabin seeking somebody who has been identified to them as a senior Nazi officer. They end up shooting and killing him, and as they ride away from the scene they are left convincing themselves that they did the right thing; however, it's never really resolved if the guy was in fact an officer, a patsy underling, or altogether innocent.
Obviously, the victim in this story was not a collaborator, but I'm left to wonder about what information led the lady to believe he was. Certainly, if there was information suggesting he was, that should have been vetted in a legal proceeding, but without knowing what all this woman had been through at the hands of the nazis, and what evidence she had to believe her victim was working with them, I'd be hard-pressed to condemn her outright.