This isn't insanity, either. Rodger is mentally stable throughout his life, but virginity naturally becomes increasingly frustrating as time goes by. He has poor social skills, which despite counseling will never improve, but he's not schizophrenic and does not slip into psychosis or anything like that. At worst he might be a high-functioning autistic, but even that is unclear. Maybe he is just suffering from social anxiety. The most insane part of the book is when he thinks he will win the lottery and dabbles in the law of attraction, but he gets over that and there are no magical beliefs guiding him on his way to revenge. He is rational and calculating and meticulous, even planning his suicide well in order to avoid capture and imprisonment (use two handguns with a spare in case one jams). There is absolutely no doubt that the rampage is caused by sexual deprivation, and up until the end he holds out some hope that he will get laid after all and cancel the whole retribution thing. As long as he can't have sex, he is determined to destroy some of the happy sexually active people around him. Girls who pick other men and the lucky men who get laid are targeted equally, and he even plans to kill his housemates and brother and step-mother.
You either have to experience celibacy yourself or read this manifesto to comprehend how dangerous this sort of man is. While reading, I was struck with the realization that he is at least as smart as you, and he is going to use that intelligence to kill you. This means he has a good chance of succeeding, which he did. There is no "treatment" for this condition (besides getting sex), because the incel is just as smart as the psychiatrists or cops or whoever tries to intervene, and so he will anticipate their moves and thwart them. The close shave with the cops when they almost search his room is chilling, but here he demonstrates that he is indeed sane and from then on he sleeps with a loaded gun nearby to ensure he will get in at least some kills if they come back.
The monster virgin is a true product of our sex-hostile feminist society, which denies that involuntary celibacy can ever be a problem and is more concerned with constructing the bizarre lie that women can be rapists than acknowledging the basic needs of men. You reap as you sow.