1. If you could steal something from someone you had strong feelings against, and not get caught at all, would you do it? (I'm talking something big like his creditcard)
The quick answer is no, but longer answers might lead to a yes. For example an extreme personal situation might coerce me into a position of stealing. The old example of the man who steals the medicine he cant pay for at that moment in time to save the life of his daughter'.
Additionally if the reason i had strong feelings against the person was because the wealth he/she had was obtained through slavery, coercion, exploitation or some other revolting means, then my taking of it might be more justified, especially if im having a robin hood moment.
2. If a criminal who murdered your family vowed and in fact even somehow showed evidence that he would not murder anymore, would you punish him or let him go? If punish: How is this different from revenge?
If i take the philosophers armchair and think about punishment in this case, i dont have too much to go on. No doubt i would like revenge, and perhaps that is sometimes just. However one aspect of punishment from a deterrence justification is its effect on other people and potential murderers. Even if one could guarantee that this individual would not harm anyone else again, seeing the anger and revenge in people affected by such crimes, and the punishments that go along with murder is a very strong deterrent and control of behaviour in other people too. Thats worth something.