I've had quite a few discussion with Mormons on this issue. In my experience, they don't atempt to deny that it happened.
Most admit the truth of the incident, with the caveat that it cannot be understood without studying the historical context, which is a perfectly valid point. They seldom seem willing to make this same concession when discussing the attacks on Mormons in MO and IL.
Many then attempt to establish a moral equivalency between this event and the mob attacks.
A few of the more extreme Mormons try to make the attack out to be some kind of self-defense against dangerous invaders who were murdering, pillaging and poisoning their way across UT, with intent to bring back an army from CA and kill all Mormons.
Unfortunately for this argument, self defense is a little tricky to claim when you kill dozens of women and children.
Which doesn't stand, of course, because of the horrific nature of this particular event. One doesn't just gun down 120 people like this. As bad as Wounded Knee.
While I don't condone what John D. Lee and others did to the Fancher party, some members of the Fancher party DID make those very threats. They boasted of their participation in the murders of innocent Mormons in Missouri, and promised to bring more of the same on the Mormons in Utah. Such threats and boasting remove those making them from the "innocent" category.
Imagine Osama Bin Laden traveling through your neighborhood bragging about 9/11, pointing at you and saying "You're next infidel!". You would give him a glass of goat milk and wish him well?
But not ALL of them - some they 'adopted'.
There is a moral equivalence between the two. The perps of both atrocities should have paid with their lives for their crimes and John D. Lee was deservedly executed at Mountain Meadows in 1877 for what he did.
John D. Lee, seated next to his own coffin, just before his execution for his role in the Mountain Meadows massacre.
We can blame all non-Mormons for mob attacks upon Mormons and we can blame all Mormons for attacks upon non-Mormons and try to justify the murders on both sides or we can blame the perps and condemn the murder on both sides without excuses.
That said, it is curious that a historical event most Americans have never heard of is made into a movie just when one of the GOP Presidential candidates happens to be a Mormon.
By contrast, nothing was heard about the Mountain Meadows Massacre when Democrat Mo Udall was a Presidential candidate in 1976 even though John D. Lee was Udall's great-grandfather.