Big Smoke and Rider and Tenpeny
Three members of the Ballas killed Beverly Johnson (Carl's mother), as seen during The Introduction. They were, however, on orders from Big Smoke and Ryder, as seen during the mission The Green Sabre. Frank Tenpenny and Eddie Pulaski of C.R.A.S.H. were more than likely involved, possibly supplying the car used in the killing, as Tenpenny is seen with the car in the lock up during The Green Sabre.
Edit: please note that Beverly was killed by mistake. the target was actually Sweet, but the bullets found someone else instead.