I always saw evil as - among other traits - an abundance of selfishness and greed. IMHO, just being evil alone does not warrant the character does anything particulalry evil for the sake of evil (although being a champion of evil and in service of an evil god may change that...), but that he will be ruthless in the pursuit of his own goals, and those are all that matters.
As an example, recently I had my party enter a demiplane in the service of some oriental gods retrieve a powerful artefact. The demiplane started to corrupt the PCs and NPCs, turning them slowly evil with each failed will save over a couple of days. Only the chaotic good barbarian remained - surprisingly - neutral, the rest had become evil after a few days. Now, the characters did not give up on their quest, or went freelance, but they became more selfish, and ruthless. Minor quarrels almost led to blows, the originally chaotic neutral sorceress was plotting the death of a chinese martial artist out of jealousy, the aristocrat became possessive of all females, and ready to kill any rival, and in the end the party leader, who had taken to motivate the sorceress with slaps and blows to "work harder", traded the chinese martial artist (or rather, her body) to an evil soul-trapped mage in order to get the necessary help to recover the artefact.
All in all a good portrayal of a "smart evil" party, imho. No irrational evil acts "just because", no stupid killing of potential allies just to do something evil, just a bunch of ruthless opportunists without any scruples to take the easy solutions to any problem and a willingless to sacrifice others to save themselves.