It is not evil to want to punish a man for committing an evil crime, it is called justice.
If that punishment involves spending the rest of his life in prison and in fear then so be it.
Is it wrong to commit an evil man to such an establishment knowing he will be living his life in fear and in danger?
The problem is when having incarcerated the person and condemned him to such a life, we enjoy hearing of his sufferings and feel vindicated in our hatred and vengeance and laugh, applaud and joke at this act of barbarism in our name. This isn't anything to do with "justice" or self preservation or utilitarianism; it is the manifestation of the "feelings" that huntley exhibited in his acts in the first place.
I hate what he did since like all on here and unlike him, I have the ability to imagine and empathise with the victims (and their families), just as I hate others who have committed similar acts. I can understand the arguments against hanging and accept that a quick termination is in nobody's interest for a whole host of reasons. What I cannot do is to cheer and bray with excitement when I hear of these people being hunted and beaten and maimed, because to do so would mean that we're not as far from their way of thinking as we would wish.