You should warn people if someone is dangerous.
Lana had been drinking at a party, and was sexually assaulted by Clark, one of the men at that party.
Lana doesn't want to be around Clark ever again.
Lana tells her friends what happened to her, they spread the word that Clark is a creep, and he's not invited to any more parties.
Lana doesn't tell anyone about the assault because she is ashamed, she stays home where she feels safe instead of going to parties.
Eventually, Clark leaves town because none of his friends will talk to him any more.
Clark has gotten away with assault, and goes on to assault more women until someone finally turns him in.