I don't believe in the death penalty and I don't believe in vigilante justice. Bottom line, regardless of the criminal in question or the nature of their crimes...whether they happen to be mass-murders or deranged racist clansman. I believe people who harbour insidious (and evil) beliefs that involve harming or killing other human beings and who plan to act on those beliefs should be put behind bars for a very, very long time. But I don't believe they should be killed by citizens who decide to take the law into their own hands.
From the sounds of it, this was a supremely fucked up and evil human being and I don't doubt that many people will miss him on this planet. But I feel that as a result of his actions, he deserved to be put away and made an example of via the law. If we take that process away, justice can't be counted on and the system breaks down as people begin to feel this kind of vigilantism is acceptable. I know this is just one isolated example, but I don't believe in making exceptions in regards to the way we deal with toxic and hateful individuals such as this man in the story.