Heh, that's a good argument for atheists to use. I've actually done that one before, and have yet to get an answer that fully makes sense.
One is that God wanted the person to die, and come to Him. So He did this thing and sent the murderer to kill them.
But then you have the problem of punishment, because if it was God's idea then the murderer isn't at fault for it and shouldn't be blamed. In that case, you could point a finger at God.
So if you want to follow that logic, murder is not a sin because it must all be part of God's plan in some way. But that means that:
1. The Bible, which says murder is a sin, is wrong.
2. We don't really have free will, merely an illusion of such.
What it comes down to, is what you want to believe in. You can forgive the guy, and try to get him pardoned because God wanted him to kill the woman. You can let him rot in jail for commiting a sin, which must have been part of God's plan.
The first is more than any human can ask of oneself, but the last requires you give up the ideal of free will to an exent.
Have fun with that, and I hope I was of some help.
You walk by me every day, but you have no idea.
I sit there, in the corner all alone.
If I got up, would you know then?
What would be your last thoughts,
As I slit your throat and drank of the sight of your blood.
Would you think, "Who is this person?"
Or would it be, "I should have kept walking, today."