Although you weren't specifically asking me, I've seen alot of kids who abuse other kids. Again, these kids have invariably been abused themselves.
Now how about this one. If you ever run a survivors group (i.e., survivors of sexual abuse) with adolescents, ask them to raise their hand if they in some way or another enjoy the abuse when it happens. You'll get quite a few raised hands in my experience. It seems that even though the attention they are receiving is terrible by adult standards, this often the extent of the "loving" contact and attention they get from parents, adults, etc. Consequently there is something about the encounter that they want and need, namely adult approval and contact. This conflicting emotions about the encounter (hurt and reward) can be extremely confusing and shame inducing in these kids. I've found that they are usually so relieved to hear others have these feelngs. I hope I explained this clear enough.