Paul answers this 

Why are we to blame if God uses our unfaithfulness to accomplish His purposes ?

But if our unrighteousness brings out God's righteousness more clearly, what shall we say ? That God is unjust in bringing his wrath on us ? (I am using a human argument.) 
Certainly not ! 
If that were so, how could God judge the world ? Someone might argue, "If my falsehood enhances God's truthfulness and so increases his glory, why am I still condemned as a sinner?"
Romans 3:5-7