The historical Brigham Young was pretty hardcore.
He talked about killilng thieves on the spot, instant justice, judge dredd style, but it was also to sort of purify their souls while sending them to the afterlife. It was connected to his doctrine of blood atonement.
A white person marrying a black person was reason for blood atonement. He didn't really fit our current image of Mormons.
The defense would probably be along the lines of it being a violent time, but he didn't exactly discourage the violence. That was pretty aggressive compared to other sects or fringe religious groups of the time, I think.