Well, your question is somewhat misconceived addressed to a Catholic. We don’t think of right and wrong as something God decrees so much as right, or righteousness, being intrinsic to God’s nature. Otherwise you would end up with this conundrum: does God forbid something because it is wrong (in which case there is a law over and above God), or is it wrong because God forbids it (in which case right and wrong are arbitrary–God could have made it another way).
With that in mind, I will try to address your question. As Catholics we understand that right and wrong exists independently of what we think about it, and that what we need to know about right and wrong has been revealed to the Church. Nonetheless, we consider it a worthwhile undertaking to try and understand the nature of right and wrong. So here we already know what the right and wrong thing to do is in the situation presented. The question is: why? The trick is to come up with an answer that doesn’t simply restate the already known conclusion, such as “it is opposed to the law of God.”