I guess because the letter to lay down arms at the end came from the Hatfield side, and the special needs Hatfield kid was hung (truth), it might seem so. But, damn, McCoy had all four of his sons and one of his girls killed, his wife crazy and another daughter dying young after giving birth to a Hatfield out of wedlock. The worst was the three of the sons tied to trees and executed in cold blood with no trial. I thought that was the most brutal act of the whole affair. In light of that, the posse from the McCoys was understandable although it killed a lot more than arrested.
IMO, neither side looked very good. Johnsy Hatfield was maybe the most likable, but he was rife with moral faults.