Going back to the episode real quick...I just rewatched it, and clearly Jimmy was just a dumbass. He saw Rick on the roof and Rick directed him where to go so that he and carl could get on top of the RV. So he knew they would be on the roof. So why was the bloody door unlocked? That is the ONLY way that the Zombies could have gotten in (especially given that the door swings out). The only explanation for his death is that the door was unlocked so someone fraked up big time.
That was the only real problem that I had with the ep...well that and T-dog's horrid u-turn.