He doesn't come off as the least bit psychopathic.
He comes off as perfectly sane, and extremely angry. He describes some psychopaths in his manifesto, and a large amount of them are wearing badges and toting guns.
He murdered the people because he has nothing to live for after they destroyed him for not falling in line with the good old boy network. He had a perverse sense of justice for a group of individuals who act like they are immune to it.
I'm fairly confident that at least 90 percent of the manifesto was accurate, especially considering it isn't anything new.
I actually agree that he doesn't sound especially crazy. Of course, that doesn't really matter; he went on a kill-crazy rampage and I'm glad he burned on earth before burning elsewhere.
As an aside, phychopathy is kind of a vague term. I mean, going by the criteria, most teenagers I know are psychopaths:
1.Superficial charm and good "intelligence"
2.Absence of delusions and other signs of irrational thinking
3.Absence of "nervousness" or psychoneurotic manifestations
5.Untruthfulness and insincerity
6.Lack of remorse and shame
7.Inadequately motivated antisocial behavior
8.Poor judgment and failure to learn by experience
9.Pathologic egocentricity and incapacity for love
10.General poverty in major affective reactions
11.Specific loss of insight
12.Unresponsiveness in general interpersonal relations
13.Fantastic and uninviting behavior with drink and sometimes without
14.Suicide threats rarely carried out
15.Sex life impersonal, trivial, and poorly integrated
16.Failure to follow any life plan.