6. Going Back For Merle Dixon.
This. He should have shot Shane when he was going psycho right before busting open the barn. And risking people's live for Merle was idiocy.
That said, Rick is not as dumb as Lori was.
I don't blame Rick for not killing Shane earlier. They'd been friends since childhood; I understand why he gave him every chance.
That dumbbell Lori couldn't even manage to make a pancake.