To #20. Lanza’s killing his mother and then the teachers/administrators and school children makes a lot of sense to a damaged mind.
His mother was the prime target, but then he “projected” his hatred for her onto anyone who worked with her or were in her classes (or any classes at the school). Thus he kept “killing” his mother eventhough they were totally innocent and not socially connected to her.
Only after he did his killing did Lanza realize that he was the last living link to his mother so he had to kill himself in order to sever that emotional/mental link.
The psychological term is “projection” meaning projecting one’s own fears, hates, likes onto others who may have been close, rejected them, were in the proximity of those people, or who had a direct connection to them.
Paranoid schizophrenics do this all the time, even if the people, events, and perceived threats, insults, turn-downs never happened in reality.
He got his revenge, on anything that mattered to her.