Nick had a post at Troll in the Corner a while ago titled Evil: A Do-It-Yourself Guide.
The gist of it is that evil characters are likely that way for a reason, and most often do have self-control. This means the evil acts they do likely have a reason (if not a one most would consider reasonable). As he put it at one point, “The Good are tempted by easy paths to power and riches. The Evil take them.”
All in all, I agree with this post… and I was reminded of a conversation years ago regarding casting charm person on a powerful evil character in order to defeat him. This is a hazardous idea.
“It says he considers you his friend!”
Indeed. It does not take away his ambitions, however, and if you stand in his way then he may be regretful, but will still act as he sees necessary to achieve his goals. He is accustomed to losing (or selling into slavery) his friends, what’s one more?
Come to that, it’s unreliable protection even when you don’t obstruct his ambitions. If the best way for him to survive a situation involves dropping an Empowered fireball right where you’re standing, he might not even hesitate if it means someone more important (that is, himself) does not get hurt. Lawful Evil has ‘accepts collateral damage’ written all over it. Neutral Evil might see it as a helpful preventative measure — if you are no longer of use and may become a threat, it might be a goo… prudent decision. Chaotic Evil might do it just to see how long you scream.
I agree that most evil characters can have self-control… as long as it is convenient, does not conflict with their goals, and is not a danger to them. When those conditions are not met, though, it might not be such a good idea to rely on that self-control.