Manga

Just found out that one of my friends is into reading manga comics. He told me about the storyline of the one he's reading at the moment and it made me feel physically sick (the story involves children raping and killing each other). I think it's the fact that he's not just reading about it but he's also looking at drawings of it that disturbs me so much. He's in his twenties so he's an adult but I can't believe he enjoys reading/looking at these kind of pictures. Does anyone else out there read these comics? I'm curious to know what the appeal is. I've never looked at one myself.