In his mind they had something to do with it, because they were a part of his mother’s world. Imagine having to live for years with a mother who saw you as her only world. Not only did she drop from having any jobs to be a stay-at-home mom for the formative years but she also handled her son’s education by herself. She would teach him her beliefs and political ideas through firearms training and survivalist ethics. That is the love of a mother.
And than imagine that the father and brother split away from you because they see you and your mother as crazy, as not interested in you. So who is left? That is right your mother and the world she lives in. So imagine the personal betrayal you would feel if your mother tries to hide the idea that she, herself, thought that you were crazy enough to be committed.
He also had personality issues. He could’ve had Anti-Social Personality Disorder, and he could’ve been taken medication that might’ve caused mania in him which increased his aggressive scope of behavior. In the end, he saw his mission as destroying his mother’s world and than destroying his mother along with it. I get it completely, however that doesn’t justify it.
P.S.: If this is a lot of the info we have, than we mostly have just speculation which is never good when we start pointing fingers. I would like to deny it, but I might as well accept that tragedies happen in a free society and to move on to take care of my life.