I don't think there is anything unethical about taking someone's money playing poker, regardless of whether it was a mismatch, a bad beat, or whatever. The whole goal is to make money and enjoy the thrill of victory. You're not responsible for his having made the decision to open an account online and start playing poker. He shouldn't be there with money he can't afford to lose, true, but (1) you don't know if thats the case, and (2) it's not your problem. People are responsible for their own actions. Would you feel bad about taking it if you found out that his net worth was 300 times yours? How do you know it wasn't? The point is- what someone else chooses to do with their money isn't your problem. You are not stealing by beating someone in a fair game in which there is no cheating. Being much better than someone isn't cheating. There is no unwritten rule that says you have to suck to accept a challenge from someone at a low-limit table. Did it ever occur to you that he was a shark, playing poorly on purpose at a low table, looking for someone weak to try to take down in an HU match? You're a good player that decided to play at a weak table- why do you think no other good player would ever do this?

By the way, assuming he was a bad player, taking a moderate amount of money from him like $200 is probably a good thing for him. Here is why: he's only losing $200 but he's learning a valuable lesson, and maybe it will stick. Maybe he'll think about this experience down the road when he's debating whether or not to make an early withdrawl from his kid's college fund to go play someone in a heads up challenge. Not saying this justifies it, but it could very well be true.