Who says I'm excluding people from my analysis? Harming people that harm me (by locking them up let's say) helps me, but harms them. The benefit is infinitley greater to the cost, in my perspective.
If you lock up your enemies, you might go to jail for kidnapping (or whatever that's officially called). Or, they might overpower you and injure you. Those are risks. So I wouldn't say it's infinitely greater.
But really, what's the benefit of them going to jail? The benefit to you is that you've got your revenge, and you feel slightly justified. The cost to them is that they lose their life, freedom, well being, possibly their family, friends, and way of life, among other things.
So is jailing somebody an acceptable activity? I'm not so sure. I don't think the penal system works at all.
But this is all really a subject for another thread. We've gone way off topic, and I'll just split it if this continues.