Playing “Evil” Characters can be fun to play as a Character Concept, from simple curiosity to unleashing pent up frustrations and emotions from real life. Also, it is not the GM’s or other players in a group’s job to look the other way or accommodate this. The behavior of evil characters often leads to less trust, resentment, and downright hostility (in both PC and the players!) if the actions of that character go against the PC group’s general outlook and endangers them or holds them back. The same for NPCs in the game. Saying ‘it is what my character would do’ has just as much validity as a town arresting and jailing and/or executing a character (or group) because ‘its what the law or king would do’. Think of the rest of the group and the fun of EVERYONE involved before letting such a character drag down the fun for all. Why are the evil? What event drove them in that direction? What do they hope to accomplish using this perspective? Character Stories can be worked out with the GM to make a workable solution.
"But, it’s what my character would do!" - These types of character aspects are not an excuse to be disruptive to the point that it constantly endangers the safety of the group or the story continuity. Find creative ways to work through situations and do not manifest them to extreme levels of Behavior for your own amusement and to the detriment of the group.
Playing a character you know any reasonable party would abandon or kill is not healthy for anyone.