It is sometimes scary to realise how many people there were in both America and Britain who actively supported Hitler in the run up to WWII. However, I think many of these were unaware of the true extent of the 'final solution' as several sources I have seen indicated that even many staunch Nazis in Germany were unaware of the true nature of his plans. The build up was so subtle and quiet that many did not realise what was going on. It was only after the war (or possibly in the last stages) that much of this information was made available.
The main problem with the death penalty is the irreversible nature of it. All you can do is apologise to the family and that means shit when their son/brother/father etc is dead for a crime they did not commit. I am equally not convinced that prison is the answer - many criminals go into prison and come out just as willing and able to commit crimes. Its not an easy question to answer at all and I am not convinced it is so easily organised into a polarised 'right wing - string 'em up, liberal - lets talk to them and understand them' debate. I do wonder if some 'make the punishment fit the crime' system may be appropriate - rapists get castrated, thieves have to pay back all the money they stole or work as a slave for the person they stole from until it is repaid, murderers get killed - but that is certainly going too far for the larger crimes (i.e. I still disagree with the death penalty).
Insanity... many people think of gibbering, cackling madmen when they hear this term. This is an inaccurate stereotype. There are many forms of insanity where the patient can be incredibly highly functioning on an intellectual level. Sociopaths are the prime exampe. These are usually highly intelligent and can plan to a very high degree. Their 'insanity' manifests in a complete emotional isolation. They do not feel the same way as the majority and therefore see no moral or ethical problems with hurting or killing another person in order to achieve thier own selfish ends. Most serial killers are sociopaths. Many other psychological conditions can cause someone to commit a crime yet maintain their ability to function.