A friend of mine who plays hockey told me that she feels bad whenever she hurts people in hockey, even though she's hurting them before they have a chance to hurt her. At first I was going to say to her that she shouldn't show them any mercy if they weren't going to show her mercy. Then I realised something; that's terrible advice. I began to realise how terrible a phrase that was. I used to agree with it really but I don't anymore.
If you're a compassionate person who no matter what would never inflict vengeance upon another person then you should never compromise who you are for anything, especially not the actions of others. As you may know I am a huge opponent of any form of physical punishment, especially the death sentence. You can't justify killing. If you kill someone because they killed someone, congratulations; you are a murderer. Well done you! Now please report to the electric chair. It's a never ending cycle of death and violence and the only way for someone to stop it is to say "Not me. I won't do that."
We should reduce ourselves to the level of others? We should kill because they kill? That's devolution, not evolution. We should stand tall, proud, and let people know that no matter what they do to us, we shall never be like them.
I hate to bring fiction in to it but look at people like Batman. He never kills, no matter what. One of the points of the Joker is that he does anything and everything in his power to make Batman kill him. To take away the one thing Batman has that makes him different. I'm not saying that I would be able to forgive someone who killed my best friend. Chances are if you did that I would strike you down in a heartbeat, but the point is I shouldn't.
Even if they won't show you mercy, you should show it to them. It is not a weakness, it is a strength.