Terrance plays into Mac's imaginary friend story only to make him suffer even more as he tries to separate them. Seeing that she protected Mac the most, this created an anger toward his brother. The reason for Terrance's constant bullying is that he felt that their mom didn't love him. The reason why his biological mother isn't around and his Father was never mentioned in the show is because his mother murdered the father for torturing her two boys and was charged with Voluntary manslaughter and sentenced to life in prison. His biological mother protected Mac the most because he was too young to defend himself.
His mother was never around, that's why the imaginary friend, Bloo was needed, to keep him company. A shoutout to Siam Cat and Kit for questioning me about mac (I know this one is quite different from the other one I did but oh well)
