What is the difference between revenge and justice? Example: Some guy holds you at gun point and wants your money. You manage to snatch the gun from him, would it be considered justice or revenge if you now rob him instead?

I don't think that justice and revenge have all that much in common. Justice is about fairness and morality: doing what is right. Revenge is about inflicting hurt or harm on someone in response to wrong suffered at their hands. But two wrongs do not make a right... acting purely out of vengefulness is rarely just at all. If someone has done something wrong then they should make it right if possible ...make an amend, redress the balance. In a just society the legal system may enforce this. But the point is not to make the wrong-doer suffer. That does no good to anyone. It is only to make things better. If you manage to snatch the gun from the would-be robber, then robbing him would be vengeful but not just.

Is it morally acceptable for the rich to use their wealth to hoard an *essential* resource (such as housing) in order to make a profit?

One's answer to that might well depend on one's general politicalorientation. There are those who think that property is theft (a sloganassociated with Karl Marx and Pierre-Joseph Proudhon). Such peoplewould tend to answer your question 'absolutely not!'. Others believethat people can and often do own things and do have a right to hoard,if they choose to: it would be a good and generous thing for thewealthy to share their resources. But they are not morally obliged to.Personally I think it is hard to justify the institution of ownership.John Locke famously attempted to do so. (I think this was related to his defence of British colonialism in America). Roughly speaking (if I remembercorrectly, which perhaps I don't - maybe someone will correct me if Iam wrong) his idea was: God gave the Earth to mankind in common. So ifthere is some as-yet-unowned good, you have a right to put it to use -mix your labour with it and thereby increase its value. Since you own your body, you thereby come toown the good....