Who does everyone think the shooter in the finale is going to be, because apparently it is someone we know?

I think it's the guy who lost his wife because she signed a DNR. When the guy came to seattle grace in 6x21 he looked so mad at Derek, I think he's a potential shooter. I have no oder clue...
I'm thinking about it now all week and I still don't got other candidates. I so can't wait till friday!!!!!!!