
Bum was simply hiding the gift for Sangwoo. Sangwoo hir hair just because that is a reaction of people mentally instabile or with so much mental pain they can't "digest". The reaction in this cases could be more physical or more like yelling, you see the physical ones often in people from Africa and we still don't know why they have a more physical way to show a huge mental distress

Yeah... Africa is a huge continent of many different cultures so this generalization is pretty strange to me. Even if large physical displays are the norm in some of those cultures not all would show this. If you've got a study you are referencing, I'd love to see it. As a Nigerian woman, I don't particularly enjoy being stereotyped.
I think you should give the writer more credit. Sangwoo could have lashed out his emotional distress in a myriad of ways. He specifically cut his hair off just a couple chapters after asking Bum if he wanted to know why he dyed it. Maybe it was just a meaningless reaction, but I think it would be far more interesting (and better character building/story telling) if it signified more than that. We're still adding layers of depth to Sangwoo's character, so I'm eager to find out if there's anything more the writer would like to reveal.
I'm interested to see what will happen with the police officer next but does anyone know what surprise Bum had for Sangwoo? Also, why was he looking into the basket with the poison and the fake nails?