Difficult. I mean, if someone is really beating a child in front of you, abusively, then yeah I think there's a point where it's your ethical obligation to stop it if you possibly can. You can't really predict or prevent someone from going over and punching a child in the head.
The corporal punishment thing seems to be coming up fairly often on this board. Basically, yeah, punching a child in the head is is too, it is abusive, and if you're in Seoul (and I think Gyeonggi) it is also illegal. In some cases it's very strongly enforced but in many other places in Korea, still very accepted/widely tolerated. If you're in a Seoul school, pointing out that corporal punishment is now illegal, and that teachers HAVE had the police called on them, etc. in recent months for stuff like this and that you're uncomfortable witnessing something illegal could be a good way to approach it. Emphasize you want to adhere to Seoul's new laws, you feel safer doing so as a foreigner and so on.
I am glad you gave some background on this kid though, because it shows that there are lots of issues going on here--students with behavioral/emotional problems, teachers who have not had classroom management training that goes much beyond corporal punishment, and a school system that pretty much leaves the teacher on their own to deal with these students without the kind of support-system hierarchy you'd have at home.
