Many Mormons do not like to hear or read criticism of their church. They believe they have a God-given right to correct what they see as misinformation about the Mormon church.
If we are to believe Mormons there was never any blood oaths in the Mormon Temple ceremony, Joseph Smith only had one wife (and if he did have more than one wife, including women already married to other men and young, nubile girls under the age of 16, it did not matter as they were only ever 'spiritual' wives) and Mountain Meadows was only an attack by Indians on a wagon train of people passing through the territory. And in any case, it did not matter, as did you know that some of those people on that wagon train were actually part of the mob that martyred Joseph Smith?
And did you know that neither Joseph Smith or Brigham Young ever ordered a group of men called the Danites (which didn't exist, anyway) to murder people? And even if Joseph Smith and Brigham Young HAD ordered the non-existent group to murder people, it would not have mattered as they would almost certainly have deserved it, and blood atonement was never preached or practised.
When Mormons tell or pass on these lies, do they know they are lying? OR do they really believe what they are saying? I remember one former Mormon mentioning the blood oaths in the temple ceremony to someone who had gone through the temple at roughly the same time as them. They were shocked when the practising Mormon denied there had ever been blood oaths as part of the temple ceremony, and challenged the former Mormon asking them why they would make up such a story? It was, they reported, as if part of the memory of the practising Mormon had been surgically removed. Unless, of course, the Mormon was just a liar? Which tends to indicate that some Mormons do "Lie for the Lord."