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 credit card)
Totally depends on why i have strong feelings against them. In the sweeping majority of reasons for having strong feelings against someone, the answer would be no.
In some, it would be a definite yes.
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?
Is evidence here indicating that i'm
ensured that they would not murder anymore?
If so, what about other forms of violence? If someone can murder a family then they can do all sorts of other violent things besides murder. If we assume that they ensured to me that they've "changed their ways", then i would want to not punish them. I'd rather let them start over and hopefully benefit themselves and others. I'm not sure whether or not i'd be able to do that in such a situation, however.
If i do punish them, then it's no different than revenge, and i have absolutely no problem with revenge. Not in general. That said, revenge in the sense that it would be for me would not be the only reason i'm doing it for.
If on the other hand, nothing is ensured, and/or nothing is addressed in regards to the possibility of other forms of violence, then i'd most likely punish them. As despite there being an evidence suggesting that they would change, it's only in one regard, and it's not even necessarily a safe bet in regards to that one thing.