Why are people insane (who don't actually have mental health problems)?

Newsday headline:

Man Tries to Steal Gun to 'Rescue Schiavo'

...Randy McKenzie, the owner of Randall's Firearms, said Mitchell pulled out the box cutter and broke the glass on a couple of display cases. "He told me if I wasn't on Terri's side then I wasn't on God's side, either," McKenzie told The Associated Press. McKenzie said he then pointed his own gun at Mitchell and ordered him to lie on the ground. But Mitchell fled out the store's back door before police arrived, he said....