The Volstead Act, the Golden Rule, and random irritating intangibles

I'm not a particularly moral person. I try to be "nice," and any injustice infuriates me, but I don't put in a whole lot of time worrying about ethics as, like, a topic of serious thought.
Got thrown into a situation yesterday where an acquaintance assumed I'd "be cool," and...well... I wasn't "cool." Upon hearing the story this morning, a buddy who knows everyone involved basically implied I'd been an asshole to turn this guy in when it's possible he'll get fired. I guess that's why I have this very After-School-Special-flashbacky uncomfortable feeling about the situation. My best friend calls me "the Zen Master of CYA," and he's got a damn good point. I frequently question my own motives, and it's a tough call. Was I a rat fink because I'm not risking losing my job for some jackass, because the jackass assumed without asking that I'd condone his behavior, or because his actions were inexcusable at a work function and just generally immoral bullshit? All of the above? And does it really matter, anyway?
I kinda think it does.