If the White Army hadn't been killing civilians, how would that have affected their war effort against the Reds?
It was because of stuff like the White Terror that hurt their cause in the eyes of the people. They were seen as no better than the Reds in that regard. Apparently the atrocities on both sides were known in OTL.
I figure that had the Reds been left to shooting strikers, working people to death in concentration camps, and blowing up churches, they could have used it all to make Lenin look bad.