Quiet Characters Evil Keeping
If a character starts committing evil actions, there will be an impact. Exactly what depends on what has been done, but among the more likely responses are a band of adventurers hired to hunt the character down. In extreme cases, they might be composed of paladins devoted to a god of justice. Most characters aren't exactly likely to take such in stride, even if they'd been willing to ignore the evil PC's offenses.
I'm not going to just slap someone down just for playing their character, but all actions have consequences, and some actions have consequences likely to encourage some level of discretion.