It's ridiculous, not ridiculously easy, and I answered it:
Anyone, including myself, who believes without a doubt that God has instructed him to do anything, will do it.
It's hard to take you serious.
First you posit that the hypothetical person believes, without doubt, that he has been instructed by God to kill someone, and then you ask "How would you kill someone in the name of God if you don't believe in Him?"
Well, clearly if the person believes, without a doubt, that God has spoken to him, then he believes in God, irrespective of his prior 50 years of disbelief.
Again, it doesn't matter whether or not the person is crazy or sane and blessed, if he believes without a doubt that God has instructed him to do something, he will do it.
You're trying to tell us that if you were crazy and believed without a doubt that God told you to kill someone, you would tell God "Go **** yourself."
Perhaps a crazy person might do so, but not as a result of a rational consideration of the matter - which is what you want to contend.
First of all, crazy people cannot rationally consider anything, and secondly the response "Go **** yourself God," is entirely irrational if you believe without a doubt that God is speaking to you.
In your simplistic view of the topic you somehow think that telling God to **** himself is heroic.
If you truly don't believe in God, then it is idiotic to tell "God" to **** himself.
Your argument is stupid and doesn't even rise to the child-like argument that there can be no God because humans suffer.
I'm sure you consider yourself a champion of rational thought, but you are far from it.
The only believers you are likely to draw into your flimsy web are bound to be simpletons and as much as I'm sure you would like to believe that all believers are simpletons, a relatively intelligent 10 year old believer can shoot down your argument in about 10 seconds.
There is nothing ridiculous about your atheism. The existence of God cannot be proven and so the position that he doesn't exist is not unreasonable.
For some reason, however, you seem to want to to express your atheistic belief in a belligerant manner. Arm-chair psychology would suggest that you are not so firm is your atheism as you would like us to believe.