In terms of ratings it usually boils down to exacerbating and mitigating factors, and one of the most significant of these is blood and gore. Batman [...] only depicts "static" blood (i.e., on walls, floors, etc.). [This tends] to diminish the intensity of the violence, which can help it to remain a T-rated game. Halo, on the other hand, is a first-person shooter that graphically depicts "dynamic" blood and gore (i.e., blood splatters, chunks of flesh, etc.). While most of the blood is purple, some of it is red, which to your point is a noteworthy distinction. All of these factors usually raise the intensity of the violence being depicted.