MMI,

I've been involved in this argument before, so I'm going to be brief and not get too involved again. As has been pointed out, this kind of argument is pointless as neither side is likely to be swayed by the arguments of the other. But to your points:
Item 1: Arguing the benefits of something simply because a majority of people, or in this case governments, agree with you is circular reasoning. Just remember: the majority isn't necessarily right, but they usually get their way. And using the argument of "civilized" behavior won't win any prizes, either. It hasn't been all that long, historically speaking, since "civilized" people found it perfectly acceptable to massacre native Americans, or to buy and sell slaves, or to view public hangings as family entertainment. The definition of "civilized" behavior changes with society.

Items 2 & 3: Insanity is a relative term. For every psychiatrist who claims a person is insane, there will be at least one who says he is NOT insane. For my money, if a person knows right from wrong and still does the wrong, he does not belong in a mental hospital but in a prison, at least. This is simplified, but I think you can get the idea.

Item 4: There were very few non-Jewish people in the world who were seriously disturbed by what Hitler was doing while it was happening! It was only after the fact that people screamed for punishment. And it more because of the numbers of people who were killed, and the horrid manner in which it happened, which disturbed people. If he'd instead only executed a relative handful of certified lunatics do you really think the rest of the world would have cared?

Item 5: There is only one absolutely certain way to insure that he will never again threaten another person. Civilized or not, moral or not, it is quite certain. There is no doubt of his guilt, no chance of a mistaken conviction. He did it, he was caught red-handed and he's admitted to it. Game, set and match. In this kind of case the only one's who benefit by a trial are the lawyers.