dear Terry...
you asked: "If mankind is wretched, depraved, corrupt and deserving of death--ON WHAT POSSIBLE BASIS can their low standard of thought and conduct be rewarded as though the offense were nothing?"...
-In the eyes of God, because we turned from His "recommendation" to follow our own course to death, we are deserving of death...NOT because we are wretched, depraved and corrupt.
-However living with and according to the law of death (apart from renewed communion with God) mankind is increasingly more wretched, depraved and corrupt.
Therefore Jesus can ask the Father to forgive "them" for they know not what they do(wretched, depraved and corrupt) and be the "conduit" for that forgiveness by renewing believers to life through Himself. (if the "offense" were nothing then it wouldn't have to be paid for at such a high price, mortal life for mortal life)
love michelle