RPGs are a great way to explore aspects of humanity that we really don't, and shouldn't, indulge in our day to day lives. They let us be the bad guy (or girl.) They let us make horrible decisions, be incredibly self centered, and live a life where perhaps we do things that we would never even consider in real life. This is all well and good, but if it's something you want to do in your next game there are some things you should really do first. Today, I want to talk about that.
Before Anything, Talk To Your Group
I'm serious. If you're looking to play a dark character you need to discuss that with your group. Yes, even if the world is already a bleak and dark place. A PC being one of the people at the table you need to be on guard against catches a lot of people off guard. They're vulnerable to being exploited that way, and what's more it isn't what they're looking to do in their social fun time. RPGs are about fun, but that means for everyone, not just yourself.
When talking to your group discuss limits (if you think you're going to brush any) and what you see your character potentially doing. Even if you don't go down the path it is good to establish what the group is ok with ahead of time. You trying something new shouldn't make someone else at the table feel uncomfortable - or worse, unsafe.
Talk To Your GM
Aside from your group you want to talk to your GM as well. Tell your GM what kind of things you're looking for, what kind of challenges you're looking to face, and what kind of reciprocation you are open to. People doing bad things often have bad things happen to them. If you're going to be going around murdering people in cold blood from ambush, is that ok for the GM to do to you? If you're going to be threatening loved ones, is it cool for the GM to threaten things you care about? You want to think long and hard about this. Some people - and there's no shame in this - aren't comfortable being put into positions you could end up in when doing this, even if they want to try being the person doing them. It's kind of like making fun of people, but with a much higher chance of bringing up mental scarring and trauma. Be careful.
Establish A Safe Word
There should be something that everyone in your group knows that brings the action to a halt if someone is uncomfortable, or otherwise worried about what is going on. When this word is said, it needs to be respected. Then, as a group, you can decide what is going on. It is also important, especially for you, to respect when others use the word if they think you might be going too far. Remember, RPGs are about fun, not about your fun vs. other people's feeling safe.
If Mentality Lingers...
This one is a tough one, but I've seen it and had lengthy discussions about it with people in my playgroups. Sometimes playing a dark character makes the menality linger in your head. Sometimes the character latches on to something inside you and refuses to go away for a few hours, or even days, after game. This is ok, but you need to understand it for what it is. You also need to analyze if this is something that is healthy or good for you right now in your life, and if it isn't, there is no shame in backing out.
A player in one of my games made a character who still personifies the meaning of "Rage" to me. The player had a lot of fun playing the character. The other players liked the character. I liked the character. However, the character's anger started leaching more and more into the player. It didn't do anything bad, it just sat there enough he was aware of it. It got to the point that two days after game he could still feel the character in his headspace sitting there and seething with rage. He opted to back out, and so we worked out a way for the character to go away. Then, before that happened, the group wiped so it was kind of a moot point. :P
The point here though is your chasing a rabbit down its hole when you play some of these characters. Make sure you stay grounded in the real world while doing it. Stay safe. Have fun.