I have seen alcohol ruin marriages and careers and land people in jail for violence - bar fights and such. Not pot for that stuff though.
Loughner was already a crazy bastard, so I'm sure pot helped his madness along. He was also messing around with some weird over-the-counter psychaedelic. I don't know much about that. He also used mushrooms extensively. If you're teetering on some mental precipice, that stuff will kick you over the edge real quick.
He had so many other things helping along - manic role playing internet games, the neighborhood jerk for a father (according to some), alcohol poisoning.
I think he was naturally crazy and, through fate and really bad choices, brought about the perfect storm of aggravating factors to bring it all to this end.
And it appears that everybody who knew him merely increased the distance from him as he spiraled down, rather than try to help him. Seriously, I have not heard of a single person who thought to reach out and try to get him help. They all just basically changed their phone numbers and forgot about him.
see my post below yours