There's actually a number of moral/ethical/personality issues at play here, which is why I found the Twitter poll interesting, and basically reposted it here with an additional twist.

If a stranger is an asshole to you, do they deserve your help when in need? And if not, if you can take advantage of them later, would you?

If a stranger tries to scam you, do they deserve your help when in need? If not, is it okay to steal back from them if you have the chance?

Does someone's criminal actions against you make you change normal ethics regarding how you treat them?

Is stealing ever okay, even from a known thief who tried to steal from you?

I've always believed that there aren't absolutes in how you treat other people. I believe you should establish high standards how you treat typical people, and then modify them up or down depending upon how that person has treated you. If someone is close to you, then you do extra favors for them which you wouldn't otherwise do for strangers. If someone has tried to harm you, then not only don't they deserve favors, but you're justified to harm them back in the same way.

In an extreme example, let's say a guy pulled a gun on me and started shooting at me. However, he missed, and I start running away. During the chase, I hear him fall to the ground and the gun fly out of his hand. I then look back, and the gun is right next to me, and he's on the floor, hurt and unable to get up. 100% I'm shooting him here, even if I know he has no other weapons on him. Some people couldn't do this. I could, and I would feel no guilt about it. 100% I'm killing someone who tried to kill me, even if he failed and is of no current threat. However, I am otherwise a very nonviolent person, and would never physically harm anyone who hadn't harmed me (or my family) first.