What do you think of the insanity plea?

Can someone genuinely not know right from wrong, yet use a computer to research bombs to the point of making one without coming across many, many articles and pages showcasing the horror and destruction bombs cause? Is it feasible that someone can amass guns and ammunition over an extended period of time, only to go into a dark movie theatre and start massacring people because they can’t control their impulses?

Do crazy people know they are crazy and just can’t help themselves?