he was suppose to become the worst possible killer imaginable to see how far an expert "ye old" warrior could fair against modern weapons and training. to do this god hid talents for anything and everything crime related deep into Ryouma's soul as part of the "reward" for the trial, btw the other world's gods didn't catch these hidden talents at first and only become aware of them much later. god then purposely made his life miserable
in an effort to make him snap. But the whole experiment ended as a failure due to ryouma's self restraint, despite earth god meddling with his soul to a point were Ryouma is now resistant to gods meddling with his soul. so fuck god (AGAIN)