the immigrants learned that the Indian raid of their cattle was supported by local settlers. This occurred when, after the initial Paiute raid, two of the immigrant men went out looking for help and discovered members of the local militia had organized the Paiutes to perform the raid. The militia members then killed one of the two immigrant men, but the other made it back to camp to, as presumed by the locals, inform their party of who killed his riding partner and who was really behind the raid. Haight wanted to eliminate the rest of the immigrant party because he feared that they'd spread the news to California where another detachment of the US Army was located, thereby creating a scenario where Utah was getting attacked on two fronts.

Brigham Young approved of cattle raids during this time, but was specific that they not harm any person. Lot Smith famously harassed the army's cattle to the northeast without any casualties, and the native Americans were allowed to harass immigrant trains (take their cattle) going south without settler interference. This was part of the effort to convince the US government that they needed to leave Utah alone. The Mountain Meadows raid was unique in that it was organized by the local settlers, which the Fancher train discovered, which then led to the fateful and horrific decision to eliminate the immigrant train.

There is no evidence that the locals did it because they thought Brigham Young would be okay with it. The courier the locals sent to BY was intentionally vague about their intentions in this regard, and there were several other immigrant cattle raids by Paiutes during this time period where locals did intervene to stop the raid, and not a soul was harmed. And, as others have pointed out, John D. Lee and others lied to BY about what happened, pinning it solely on the Paiutes.

As for the rhetoric, it was very heated and was intended to get the locals ready for what the leaders in SLC thought could turn into a violent conflict with the US Army (and not with immigrant wagon trains).