You're not alone. I find Stan one of the more complex characters in P&A. He doesn't fit neatly and consistently on one end of the ethical scale, relative to other characters. He's harboured bad attitudes, such as his former grudge against Penny, and his "success at any price" drive. But he's not a fundamentally bad person as, for example, Cyndi and Tharqa are. Those two have no conscience nor sense of remorse whatsoever. Stan has both, as we see during "The Popsicle War" when he comes to regret having mistreated Michelle, helped Karen's clique bully people, and misjudged Penny's character. Also, Cyndi in particular delights in seeing people miserable (or worse), whereas Stan generally wants people (whom he has nothing personal against, anyway) to be happy. And unlike Cyndi or Tharqa, who had "in-name-only" friendships, Stan has, after Rich took off, maintained a genuine friendship with Jack.
Conversely, although after his "heel realization" during "The Popsicle War" Stan did join the "Peacies" in their efforts to build a fairer, kinder, more inclusive social environment, he never reformed as fully as, for example, Jack did, going from being a petty street thug (if not an especially mean or dangerous one) to a devoted (if sometimes insecure) boyfriend and a promising artist, and (as far as we know) succeeding long-term in keeping sober. Rather, although Stan learned not to be an out-and-out malicious person, he never managed, in a lasting way, to be a truly altruistic or loving person. His need to get ahead and stay ahead will always come first for him.
Finally, Stan is fascinating because sometimes he's very clever and savvy at reading his opponents (in elections and otherwise) and staying one step ahead of them, whereas other times he blunders right into a trap which he should've seen coming.
"Life doesn't wait forever." --Lisa Winklemeyer