Responses:
1) Yes, absolutely you should behave in such a way that other perceive you to be trustworthy... until it suits you to not be.
2) I think this is potentially an inborn weakness in many people... similar to eating disorders... you want people to think you're thin so you behave totally irrationally and harm yourself... being perceived as "good" is useful, but if you tie yourself up in what other people think of you then you end up handing over your satisfaction to others which is a risky proposition. Avoid it at all costs, work to minimize those emotions if possible.
3) I agree... you'd want everyone else to follow the rules, be nice, fair, etc... but why play along when you have the opportunity to gain more than you might lose?