Ousama Game, Dead Tube Recommendations
Similar scenario: Teens find themselves having to do unethical things because of someone or something that causes them to do so. In Ousama Game, it's a "game" transmitted by SMS, and in Dead Tube, it's an online video website.
Both mangas assume that any character can die at any time. The pledges they must make are often very unhealthy and are most of the time related to death or sex. So, don't read while eating.