Only going off of what I saw in the documentary:Surprised more people are blaming Courtney and no blame on his mother and even his father to a certain extent.
I.would put all of his psychosis on the fact that his mother basically threw him out like a piece of garbage. That manifested when Courtney was "thinking" of having an affair
The mother/father (moreso the mother) abandonment are the root of his issues clearly.,...however once Kurt put all his trust in Courtney and he thought he had the family he had always wanted....Courtney allegedly cheating and enabling behaviors made it easier to blame her. Also, I'm sure there's a bit of "she ruined Nirvana" vs. Kurt's parent's just ruined a kid in there too.