hypothetically, let's say cain is a huge narc faggot, who believes he is unique and special even though he works at some shitty fast food place. seeing a narc of similar proportions look down on him would outrage him, and bring him crashing back down to the reality that is, he has to make/serve people's food which probably already doesn't sit well with his undeservedly large ego.
this woman may also have had something cain wanted. in this case, it was probably her vagina. so he took it out on the 6 year old, a defenceless and weak opponent who had already become a threat to him by being female and younger.
but yes, my point was that hurting a child does not always affect the mother the way you think it will.