What Timothy McVeigh did was the most extreme offense against God, and it is true that no mere man can forgive what has offended God Himself.
But Jesus is the man who is God, the God who is man; and where there is repentance and a broken, contrite heart, He can forgive.
You (and I) have no idea how much greater God is than anything we can conceive. I truly hope that McVeigh truly repented, even if it means he has to be in Purgatory until the end of the world. I will pray for him.
My sins alone were enough to put Christ on the cross...