The “Dark Side” is all about usage.
It’s attempting to bend the force to your will rather than living through it, and letting its will work through you.
That’s why the cancer analogy works well. Because if we’re all “cells” in the body of the force, dark side users are cancer cells that steal power from the whole to fuel themselves.
Lore varies, but it is possible to be an evil “light sider”, though the closest a dark sider can really get to “good” is pragmatic selfishness and not overtly evil.