Oh I doubt that?
Just watched this again recently. Early in the film I find a line a bit puzzling.
Caine was praised by his grandparents for graduating high school, a week later was shot, then took an undisclosed amount of time to recover. So O-DOG pays a visit, at which time the grandfather sits both Caine and O-DOG down and quotes the bible passage "thou shall not kill."
Caine: I ain't never killed nobody
Grandfather: Oh I doubt that. And Kevin (O-DOG) I've heard stories about you.
Later in the film the grandfather kicks Caine out of the house after he stomps someone in the front yard.
So in between being proud of Caine and kicking him out of the house for stomping someone in the front yard, the grandfather sincerely believes that his grandson has killed someone? Given his serious moral and ethical compass, it seems highly unlikely he wouldn't have kicked Caine out of the house earlier if he was that certain Caine had killed before.
The lines would have been better arranged if O-DOG had denied the accusation and the grandfather responded "Oh I doubt that" and then the other line directed to his grandson.
Doesn't detract from the film. Just a curious decision in the script in my opinion.