Why do retaliators get in more trouble than instigators?

Something I thought of recently. Why is it that when someone tries to retaliate against someone who is picking on that person, the person gets in more trouble than the person who start it? It doesn't make that much sense. If anything, the person who started it should get in more trouble than the person trying to stand up for himself/herself or at least fighting back. Why is this society so backwards like that?