A chick is not aggressive, not at 2 weeks old. Barred Rocks are not a breed known for aggression. However, if you call being near the top of the pecking order aggressive, you may be on a different page than I am. I've never known my BRs to be pushovers in the flock. That is not aggression-that is their pecking order, their society.
That said, it is possible that hatcheries are occasionally propagating aggressive strains that are not indicative of their breed, like they've done with Delawares and some others. It's not a known breed trait, though.