You seem comfortable with glossing over the fact that he killed a whole bunch of people.
The issue under consideration was personal agency, about how quickly or easily people are able to change themselves.
The multiple horrific murders, the manifesto etc are an indication that this person had some pretty severe personality difficulties. The consequence of his severe difficulties, was a severe and horrific crime. So: how culpable was he for who and what he was?
Many if not most of us operate with some type of personality difficulties. I mentioned shyness, social anxiety, etc. Things that affect our lives, things that might hamper us from doing all the things we'd like to do. Difficulties that we'd rather not have.
The personality difficulties that most of us face, aren't severe enough to lead us to commit mass murder. And yet, as most of us can attest, it is very difficult, if not impossible, to rid ourselves of these difficulties. I think most of us can relate to how hard it is to do so.
I think inside, we know that the reason we find it difficult, isn't because we're dicks, or we're bad people etc. It's because it's so difficult to change who we are.
This person had personality/emotional difficulties much more severe than most of us will ever face - but we cast moral blame on him, because he didn't somehow do something to make himself different, to take a different direction, before he got to the murders.
If it's difficult for us to change, why would it not also be difficult for him?