There is no point in trying him on the evidence if he pleads not guilty by reason of insanity. That means he admits he did it, but was insane at the time so not guilty.
An insanity plea means the trial will be about his mental status at the time of the criminal acts, not about whether he committed those acts.
The comments on this thread are disheartening. The perp is pleading guilty and may get the death penalty. Water-boarding and “truth serum” a U.S. citizen is against the constitution especially someone pleading guilty as charged. This could very well be a test case. If so, it could soon apply to gun owners, tax dodgers and just about any crime the government chooses. Does anyone really want to travel down that road.