I've roleplayed evil characters before. Well, that's not quite true. I've redefined evil archetypes to fit in with the rest of the characters. We tend to have parties that are on the selfish side of good: they'll help the downtrodden and oppressed, but it's much better if there's monetary gain involved.

The first or second Star Wars that I played was a former TIE fighter pilot. We were all Rebels, some more reluctant than others. My character was heavily pro-Empire. In my view of the universe, the Empire is a progressive government that has provided growth and stability (this was based on episodes 4-6 and some novels I've read, my character doesn't know everything).

The problems in the Outer Regions is a direct result of corrupt officials, the Emperor is of course blameless. Emperor Palpatine would never condone the actions that have been done in the name of the Empire, But he doesn't know the true situation, otherwise he would have done something about it. My character believed that joining the Rebellion would help the Empire. By bringing attention to the corruption in the Outer Regions, he hoped that Emperor Palpatine would notice and rectify the problems.

So I had a character who was pro-Empire (like me) and who could also function as a Rebel. Some of the players in my group never seem to reconcile a character to make them fit in. Sometimes a character is just too annoying or works against the interest of the party. My choice is to change the character a bit while keeping their basic personality and motivations the same, but other players can't do it, so they don't have as much fun while those characters remain in play.

One of the characters that I'm currently playing (in AD&D) is Chaotic Evil. We don't really care too much about alignment anyway, but she would still be rather incompatible if played as an extremist. My belief is that the alignment system is more of what you can't do than what you will do. So Chaotic Evil doesn't mean you have to do Chaotic Evil things, it means that you don't have to do Lawful Good things.

As an evil character, you don't do evil things just because you're evil. There should always be a reason. So my character is quite loyal to the party because they provide friendship and security. She would betray them if she really had to. But you can only betray friends once; there's a lot of investment in your friends so you're not going to alienate them on some whim.

Given all these, I have one of the more good characters in the party. At least outwardly. She can be flighty at times and a bit unpredictable, but she's not malicious or cruel or overly greedy. My character has trained evil NPCs for free and has given an evil dragon a few thousand gold to help start its hoard. But otherwise she's not in the "evil must rule the world" routine since that would make the world more dangerous to her.

So those are two of my inappropriate characters that I've fit into the general flow of the campaign without disrupting the campaign. Players have to keep in mind that playing someone everyone else will hate is no fun for the other players. It's ok to play an unusual character, but it's your responsibility to integrate that character into the party.