Every point that can be made has been made so I'll spare you the redundant text about what's wrong, right, morally acceptable, etc.
I will say this though:
I think the main reason why you (RoninPsylink) are getting so much backlash is because of your own reputation.
For the record I don't know you(RoninPsylink) or Darkstar, but I have heard of you many times in the game and it's been all negative. (I'm sure you are a good guy and I'm not saying that sarcastically)
I'm not going to say what I've heard because they may just be rumors and I'm sure there are plenty of people that say positive things about you.
My point is:
Reputation matters in this game and to slander someone over your own mistake is not cool. Ethically it would have been nice for him to give the money back, no doubt, but that does not justify your mistake.
~B's 2 cents