What I'm sort of wondering is, maybe it was indicative of a pattern of behavior and he was habitually using diphenhydramine recreationally. It would certainly explain how batshit crazy he would eventually become. Unfortunately there's really no way to know how often he used it. Plus, diphenhydramine isn't a notable drug of abuse for recreational users, with most reporting that it's unpleasant at best and others saying the experience was terrifying due to the delirium and realistic hallucinations, auditory illusions, etc. If he did use it recreationally, it's probable that he drank alcohol with it too considering most of his later videos, he's seen nursing a bottle of Miller Lite.My theory is he used the DPH as a jerryrigged version of a tranquilizer like Valium to calm himself before the shooting. According to Erowid though 300mgs is considered a somewhat strong dosage for even recreational use so he must not have done his research. It's quite possible he was seeing things like you said during the shooting which makes it even creepier.
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Having read the experience reports on Erowid, if he did use it often, it might be that his 'connection' with Danny Phantom characters and his own OCs could have been manifested by it, though I still think the underlying issue would've been that he was batshit insane and isolated. If he did use it to keep calm prior to carrying out his plan, it really strikes me as pretty stupid of him. He could have fallen asleep, plus it has an effect on motor skills, coordination and reflexes.
It's pretty interesting either way though. For some reason I'd really like to know what his parents thought of all of this. Not that they'll probably ever go public, and I don't blame them. They probably want to forget he ever existed and I guess I don't blame them for that either.