You sort of change the scenario midstream. First I haven't killed him today, then it hasn't been that long? Anyway...

I would have said kill the mage, and then afterwards give them helpful hints. e.g. "How dare you show such contempt for me that you do not even cloak yourself in the simple spell of invisibility that would have completely saved your life? I hope you'll treat me with more respect now that I have proven my strength." Hell, do that after the first time you kill the guy if you can tell he's that green.

But I'm more of a #### than Twist or Kasty, and their answer is also good. :)