
I think he was fueled by impulse, thrown off by the resemblance this woman carried and the situation now presented to him. It's not completely logical because of the many ways the snow on his eyes could fail to prevent seeing the murder but if the boy woke up, I imagine he'd be in a state of confusion, unaware of where he was before he was knocked unconcious by the fall. On top of that, he's most likely sustained injury and therefore less likely to move as much or as fast as he'd like to tear the goggles off. Crushing the woman's windpipe can't have taken that long either. Maybe Sangwoo believes visual triggers aren't as influential as auditory ones, especially those you may not be able to recognize the sound of. The kids already in an unexpected outcome of this skiing trip. Either way I'm sure the boy will be traumatized. It's a small mercy to not have seen it but I don't praise the action in the slightest because it's made through abuse born psychosis. It's terrible circumstances.
All of my ideas are theoretical anyway so that was just my interpretation.

I don't think that's what matters to him though. He could've blinded the kid for all he cares. I think he's "protecting" his mental state in some fucked up way because he believes that's worse pain than physical. I could be wrong as hell though because I don't have enough information to get in the mindset of this particular serial killer.
why the fuck did he cover up the boy’s eyes with snow