*SPOILERS* I think Dae-su was a bit too hard on himself

I agree. Just watched it again and, yeah it would be hard to live with, but they were manipulated into it.

The other guy however! I first saw this a few years ago, and I somehow thought the rumours turned out to be untrue, which may have explained if not justified his plan. Seeing it again, obviously they were true, so if it was a cultural thing, the 'villain' would surely have topped himself years ago judging by, as you say, Dae-su's OTT reaction.

Yeah, I agree. I loved the movie until the end. Why did he start acting like such a bitch? He was tricked by that stupid creep. He should have ended him. The other guy knowingly did what he did to his sister. Which is fvcking disgusting.
Really awful ending.

The reason he reacted that way was to keep the villain from informing his daughter about the truth. He begs and cuts his own tongue out to appease the villain and hopefully win enough sympathy so that his daughter would never find out what happened. If he kills the villain, he knows that his daughter will open the box. It works, btw. She never finds out bc the villain says to leave the box closed.

He wanted to cut his tongue so his daughter doesn't find out that he banged her.

The thing is, Dae-su wan't even the one who spread them; It was presumably his brother. Dae-su didn't even know the two were brother and sister (that was my impression, anyway). It was Dae-su's brother who realised this after he was told. And then presumably spread it everywhere.

Dae-su didn't give me the impression what he just witnessed was anything more than two people getting it on. The way he told his brother.

I could understand his reaction. Dae-su wanted to protect his daughter, the only family he had left.

The tongue thing, I have a completely different take on that scene. He didn't cut his tongue out to keep the secret to himself, or because he talked too much. Like some pointed out, he didn't spread the rumors. He told one person and it took off. There was one rumor in particular that made Woo-jin's sister lose it, that she had become pregnant with her brothers kid.

Maybe I'm reading too much into it but Woo-jin Lee explains to Dae-su that she had developed a condition called Pseudocyesis, a mental illness in which a woman becomes so convinced that she is pregnant that the body creates the symptoms. He says her period stopped coming, and her belly swelled up. He says it wasn't his dick that got his sister pregnant, it was Dae-su's tongue. Chopping it off is basically him castrating himself, accepting it in order to spare his daughter of the truth.