I think the most basic one is just that it links with Sangwoo's memory with his mom. If you look at it allegorically you could see it as sin or the motif of sin and death. I think what he said about the apple being red and the old woman saying that the apple must be good since its red, goes back to one of the theme of this manga, which is appearance does not represent what is on the inside, like Sangwoo to remind the audience sangwoo's ability to manipulate using his handsomeness. Because in asian countries a lot of vegs and fruits are dyed red and sweetened when in fact they are bad qualitied apples.
Also, I immediately thought of the whole Garden of Eden thing. Like that apple (offered by Sangwoo=devil) was supposed to curse Yoonbum's life.
Actually, the others aren't exactly wrong but they missed something. Remember how Sangwoo recalled that his mother was really excited about eating the apple? The same way Bum is? And then he goes on to say "She's couldn't be sane to be so excited over-" (something like that), also the apple's tough enough to make Sangwoo bite his lip hard enough to bleed. He's not just saying the apple is something small. If you notice how the old couple react to the apples, you can see that the apples actually taste horrible and once again, are too tough. But both Sangwoo's mom and Bum said they tasted really good. Ergo, the apples are an indicator of crazy (=・ω・=)
the apples could also symbolize killing, as the apple's color represents blood, and people find the apples addicting, just like sangwoo kills
They definitely taste bad, since its been pointed out that they are way too red, they cant be healthy...#biologymind ╮(￣▽￣)╭
this blew my mind !!!
FUCK! Your insight/analysis is BRILLIANT!!!
So ...Do u want to say that may be his mother was some kind of psycho??? ...Omg it's so interesting
Well, not Sangwoo-type psycho, but driven to insanity because of having to live with Sangwoo's father is moreso what I meant ^^
By the way, did anyone get the apple symbolism? I thought it was very interesting but I didn't quite understand it fully.