Counter-Example: The Operative, from Serenity. (If you haven't seen it, see it. If you have, you know what I'm talking about.)
Also, what the heck happens to Karla in the end? She sounds like the kind of person the heroes would stop...and stopping her prevents her from maintaining the balance.
See the Trope of
http://tvtropes.org/pmwiki/pmwiki.ph...ecessarilyEvil
Note that you're right in many cases, but wrong in many. It, generally, depends on how much they fall under REALLY evil.