Just to be sure I'm getting this right - he used the syringe with no needle to inject Bobby's vagina with the cocaine dissolved in water in such a quantity that it would kill her? But then why did he have to smother her with the pillow, she would die by herself..
Amos forced coke into her female private part because the mucous membrane walls of twats absorb the coke into the bloodstream quickly and very effectively. Understandably Bobby had done such a things many times before for fun (a tingling sensation results).
Looks like there were no traces of cocaine outside/in the vicinity of her female private part, though ..