Would you consider the xenomorphs from the Alien franchise as creatures that consider themselves evil, [@MelonHead]? They're always chaotic evil.

For the more interactive characters of that alignment, the less complex excuses used will typically "its fun" or "I can" or "its necessary". Just because everyone else considers their acts to be evil, rarely means that they themselves do. Many chaotic evil characters are incapable of restraint for some reason or other. When you're incapable of that, you generally can't decide whether you're good or evil.
For far better explanation and examples, we send you [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/ChaoticEvil]here[/url].

We'd suggest going with a fairly standardized plot,[@Blackmist16]. Maybe a small twist or two to make it unique, but make the essence what others have done before.