1) On "Mein Kampf" being an interesting book: Well, you should read a lot more political books before saying something like that. I read it too, years ago. I found it extremely incoherent, poorly written and thoroughly demagogic. Most of his ideas were stolen from the socialists and a nihilist like Nieztche, ironically. Of course, he filtered them through his psychotic brain and came with the mish mash some people called "an interesting book". I find it interesting only as a peek inside his brain. Not a good peek, either, as most of it is not original, and probably not what he was really thinking.
2) On why he was evil: Moral relativism was a fad more than a decade ago. It fitted well with the amoral 90's where success was measured by your money or your results, nevermind the means. So, it went like this "who can say that the cannibals are worse persons than us? We cannot judge them, they belong to a different time, with different needs. So, if we can't judge them, you can't judge my multi national company for using childs in labor camps in Philipinas to make these neat tennis shoes!" Well, as you can see, there are, there should be some moral limits. The less, the better, for we don't wan't a moral based on one religion or, what is even worse, on a minority's view on one religion.
Anyways, you don't need to get too philosophical to discover that Adolf was evil. If you really need to quetsion something, then the right question should be: "Why wouldn't he be considered evil?"
3) On so many ordinary people followed a mass murdering psycho: Paranoia and racism. Both are the best demagogical arguments and many rulers used a combination of both in past and present. "Everybody is against us" and "The different are our enemies" makes you succesful overnight. try it! It was used by almost every colonialist country in the past, and is being used by Israel and many Muslim countries now. Oh yes...USA uses it too
Of course, there are other factors. These year will be the 45th anniversary of Milgram's studies on evil and obedience. That goes in my next post because is really interesting!