I think self interest is a continuum. On one end, you have people who always put others before themselves. On the other, you have people who always put themselves before other people. A completely selfish person would be evil, because he would do things like take candy from babies if he could get away with it. Fear of reprisal might prevent him from doing many traditionally "evil" things, but if nobody would know you can be he would do horrible things. Of course, most all people fall somewhere in the middle. Most of the time, though, when people say selfish what they mean is self-centered.
While painting lines on things like good and evil can be hard at the best of times, I have to disagree with the notion that selfishness isn't a component of good and evil. After a point, selfishness becomes evil by virtue of the effects on others. Does the idle monarch and his court who feast behind glittering walls while their serfs starve qualify as evil, or merely selfish? What if they take all the food for a "critical reserve" for themselves? Are they selfish, or evil? I'd argue both.
On the flip side, somebody who sells all their possessions and donates the money to charity qualifies as selfless, and at least to Western thought, good.
As for "proper" CN, I suppose there are a variety of personalities represented by it. For instance, one could be a committed anarchist. This character believes in absolute freedom (chaos) over any law, to the point of not caring about right or wrong. It's an extreme example, but plausible in a standard DnD world where alignments get divine incarnations backing them up. Heck I met some people at college who pretty much believed that and were proud of it.
Also, there are people who simply do what they want. They drift through life never committing themselves to anything. They don't take pleasure in harming others (evil), but neither do they go out of their way to help. When asked about important moral issues, the best they can manage is "meh."
A druid who shuns the never-ending war of good and evil to tend to nature and resents the strictures law could well be CN, but more likely TN.
CN does happen to make a handy alignment for people that are actually insane, in the straight jacket sense. They may be homicidal, but I'd find it hard to pin "evil" without the ability to understand consequences.