Why claim to use the OT as a moral guide when it is filled with atrocities?

first of all....god gave us a conscience which is a moral guide......some choose to ignore it but it is there....so if it feels wrong dont do it......secondly.....there is a new testament.....which should be used as a moral guide because Jesus came and some things changed.......again.....for convenience or whatever religious b.s. some people claim, the new testament also gets ignored at times.....maybe people should refer to the original constitution to see that like the bible it was a good starting point.....but it has been changed to incorporate new ideas.....