| Mk1 |
I think people have already mentioned a lot of ways in which you can intelligently play an Evil character - and why so many players and GMs react to the idea of having one so negatively.
But I also feel some people tend to underestimate how obnoxious it can be when someone does egregiously evil (and selfish!) things quite often, and tries to hide behind "well, your character didn't actually see it / has no reason to think I'm doing anything wrong!!!" BS. Sure, you might have succeeded on the handful of checks the GM made you roll - so far - but my character isn't hanging out with you for 4-6 hours once every 1-2 weeks. They have to live with your a*!++$* of a PC 24/7 for weeks or months. So it's not "metagaming" for me to say my character doesn't trust your creepy ass-psycho.
(Although broadly speaking I agree with the sentiment that immature people with no impulse control ruin the option of playing evil characters for everyone.)