If three people plan to ambush an individual with the intention of beating that person up, and unknown to the other two the first person takes a knife with them 'just in case'. What is the criminal liability of the other two persons if:

A - though agreeing to participate in giving the beating, reconsiders beforehand and doesn't go with them or go ahead with it

B - starts hitting the victim along with the first defendant, but then is shocked when principle offender reveals a knife and stabs victim, B immediately stops shocked, and runs away

victim quickly dies from stabbing

What is the criminal liability of A and B ?