Because I am NOT perfect and I can be a huge SOB if I allow myself to be, YET I KNOW right from wrong, I KNOW it when I do it and I knwo it when I see it done to/by others, and if I, a lowly person KNOW this, then God must know it too and IF God is a superiour being then God must be superiourly moral as well and as such...
I can see the argument from morality used in support of a deistic god but not in support of the bible/Yahweh (or Allah, etc). If anything, many passages of the OT strongly defy our natural sense of right and wrong.
Do I disregard the morally corrupt God of thr OT and follow the God of the NT and pretend the OT God doesn't exist? or do I accept that the warnings of the prophets and Jesus about the scribes playing "loose and free" with the word of God means something?
Or do false presuppositions lead to false dichotomies?