If you want the details (SPOILERS FOR TRACE)
Dan fell in love with another male classmate. He forced this classmate to rape his own sister and then watch her be raped by other classmates. Then, it was revealed later in the story that he got his sister pregnant. Finally, after confessing what happened to his parents, he "murdered" his entire family (including the pregnant sister) and committed suicide.

after reading Trace I literally cannot bring myself to love Dan anymore. jfc I hate the fact that I ever idolized him the first place... it breaks my heart to even look at him but the new art style is so fire I cant leave </3