Why should you act any differently to guildies? If you're the kind of person who steals just because you can get away with it, I have absolutely no reason to expect you would act any less unethical with people you know. The only difference in the latter case is that you're less likely to be able to get away with it.
I see you do that in a random, I'm going to assume you'll go out of your way to screw the rest of us over for your own benefit. That means I'm initiating a vote kick on the spot, and refusing to heal you if the vote kick fails. I'll drop group and take a 30 minute deserter debuff sooner than assist you in getting even one more copper piece.