I suppose it depends on what you think punishment is for.
I mean, the law clearly didn't deter this man and his previous punishment absolutely didn't rehabilitate him either (I don't think sexual predators can be ever be rehabilitated, personally, maybe chemical castration is the way to go). I don't agree with vigilante justice, but I can see how that might happen in a system that is broken.
I also don't think that she went over intending to kill him - she wanted to threaten him into confessing to spare her sons from having to testify. Absolutely not the thing to do, but I cannot imagine what she must have been going through.