The book Skid Road: An Informal Portrait of Seattle by Murray Morgan begins with a description of Dr. David Swinson Maynard (Doc Maynard)'s trip from Ohio to Seattle in 1850, including excerpts from his diary. One entry from July 9th reads, "Kept guard for fear of Mormons." No additional context is given.
I'm aware of the Mountain Meadows massacre, but that wasn't until 1857. Were there other instances of violence against pioneers before then? Were Mormons a legitimate threat to pioneers?
For the curious, Doc Maynard's journal entry can be read online here.
There definitely was a reputation of Utah being a place where Mormons dressed up as "white Indians" and attacked other settlers.
As one example in The City of the Saints: Among the Mormons and Across the Rocky Mountains to California the great explorer Sir Richard Burton described a visit with the infamous Mormon marshal/outlaw Porter Rockwell. Burton had heard tales of precisely the same things that Maynard talked about. After around four drinks of the local Valley Tan whisky (evidently Porter liked his whisky neat), Porter confirmed to Burton the rumors. He then gave Burton detailed instructions on where and how to camp to avoid "white Indians." This included keeping cold camps, changing camps after dark, always camping at least a mile from the trail, and avoiding the main trail as it was "as fit for traveling as hell is to a powder magazine."
There's one specific attack I can recall that was well documented. In spring of 1857 a small party of a couple dozen men left Salt Lake City including a pair of con artists and a suspected apostate named John Torbin. Brigham Young apparently suspected they might make some mischief and particularly concerned that they might make off with some horses or cattle. The men were to be watched and if anything suspicious were to happen, "we do not expect there would be any prosecutions for false imprisonment or tale bearers left for witnesses." Brigham Young expected the matter to be dealt with. Letters sent a couple days later further down the trail to Cedar City were even more explicit. "Be on the look out now, & have a few trusty men ready to pursue, retake & punish. We do not suppose there would be any prosecutions for false imprisonments, or tale bearers for witnesses. ... Make no noise of this matter, & keep this letter safe.We write for your eye alone, & to men that can be trusted.”
The reply letter made the situation clear, "We could not discover any brand on their poor ponies, but was surprised to see them here. I received your letter by mail, & have prepared a few. A word any time is comforting. We try to live so when your finger crooks, we move."
The traveling party evidently split into smaller groups around Cedar City. A few days later a group of four men including Tobin, awoke around 4am to find their camp under fire by a group of around eight white men. They fled into the brush. Miraculously none of the men were killed, although Tobin was grievously wounded. When they returned to their camp, the men found their horses stolen and 56 bullet holes in their bedding. It is interesting to note that this happened only a few miles from Mountain Meadows. Those eight men who attacked Tobin and his likely all also participated in that massacre later that fall. Like the massacre this attack was blamed on "Mapache" Indians by Isaac Haight. Interestingly enough they failed to follow Young's instructions though, as all four men survived as witnesses.
This attack represents a pattern within the territory at the time. Around a month later a mob from Springville attacked a similar camp and killed William and Beason Parish, and Gardner Potter. And of course it was followed up by the Mountain Meadows Massacre in October. These three attacks all represent a particular ratcheting up of tension and included Mormon on Mormon violence, as well as outsiders. However they are indicative of the types of violence reported against outsider settlers.
Attacks on outsider groups generally took place as raids in the night, and usually days from any settlements. Mormon attackers generally disguised themselves as Indians. They also encouraged local Paiutes and Shoshone to attack other settlers, and eagerly traded with the natives for whatever goods were stolen in such raids. Guides for wagon trains were often specifically chosen because they had good relationships with Young and thus were spared from raids and attacks.
Most of this was taken from:
For more information on Mormon violence I recommend:
In addition to this you can read multiple contemporary accounts of violence, many lurid and likely exaggerated, in 19th century works like:
As a followup question, was there ever direct violence between the US Army and the Mormon settlers? (other than the 1838 war which as far as I know was just the Missouri state militia and not a federal force)
good past answer that briefly touches on your question and will catch up others