Well I tend to think the guy was surely slow on the uptake...his family certainly attested to that and more. But this does not excuse his actions by any mean....his intentions were pretty obvious...if or not he was alone could easily be questionable, though he appears to have functioned well enough within his fields of interest and vocation....such as they were.
They'll try the insanity defense, it's all they got but he seems to functional for it to work.
He cleans and feeds himself for one which makes him one step above the derelict outside the liquor store.