So since you've heard a lot of the Whitman's side of things, I'll talk about some of the factors from the Cayuse perspective and some of Marcus and Narcissa Whitman's background.

When Lewis and Clark came through the northwest, some Indians asked them about their religion, and requested that they send someone to minister to them. Specifically these Indians were Nez Perce or Flathead- not Cayuse. A major difference between the Nez Perce and the Cayuse is that the Nez Perce were generally peaceful, while the Cayuse were generally more likely to get violent. Additionally, the Nez Perce had had generally positive experiences with American travelers at this point (they fed Lewis and Clark and gave them a nasty case of the runs after they got over the Rockies, which you can read in their journals) while the Cayuse hadn't had any real experiences with them- but had heard lots of stories (both good and bad) about white men.

As a response, the mission board sent two missionaries- Marcus Whitman (a doctor) and Henry Spalding along with their wives- Narcissa and Eliza. These guys did NOT get along- particularly because Spalding had a romantic interest in Narcissa Whitman and she rejected him.

They disagreed over a great many things- including where to locate their mission. Spalding wanted to go to Lapwai (near Lewiston, ID) to minister to the friendly Nez Perce while Whitman wanted to go near Walla Walla to minister to the Cayuse- the major difference between the two locations (aside from which tribe) was the likelihood of white settlers to pass through the area. Ultimately they decide to split up. The Whitman mission would become a stopping point for people on the Oregon Trail, while the Spalding mission was mostly out of the way- as to if Whitman KNEW this is unclear. Basically all of this background is to say that Whitman picked the wrong place and the wrong tribe if he wanted to be successful.

Whitman and his wife arrive at the mission site and soon after, Narcissa has a daughter.They start teaching bible classes to the Cayuse but focus very heavily on "civilizing" the tribe members. Marcus did make an effort to learn the Cayuse language and practiced medicine, but primarily he encouraged them to become sedentary, start farming, wear western clothing and convert to Christianity. This is one major source of conflict, as they are basically telling the Cayuse that they need to change their entire lifestyle, while in their culture they tend to blend new things into their culture slowly.

So lets talk a little about Narcissa. She was (along with Eliza Spalding) the first white woman to cross the Rocky Mountains. Her young daughter was a pretty effective tool in working with the Cayuse- babies tend to be universally adorable. Unfortunately, her daughter drowned in the Walla Walla river when she was two years old and Narcissa became depressed and hostile toward the Cayuse, who she partially blamed for her death. She started to outwardly show disgust for the Cayuse- refusing outright to allow them in her home for worship because they were "dirty" and uncivilized. She wrote about this in her letters and journals.

Now we're at a point where the missionaries are basically insulting and disrespecting the people they are trying to save, they aren't converting very many Cayuse and the decision is made to close by the mission board. Whitman makes a trip back east to beg them NOT to close the mission, and while he's there he encourages settlers to come back West with him. Ultimately, the board decides to keep the mission open and Whitman returns. This time the primary focus of the mission shifts toward helping white settlers instead of ministering to the Cayuse- as that has already proved to be a failure. More and more white settlers arrive at the mission, which the Cayuse see as encroachment on their territory.

As white settlers do- they bring disease. Since the settlers know that there is a doctor at the mission many end up diverting their path to Oregon if there is a sick person among their party in order to get medical help. This causes a large measles outbreak. Since the whites have some immunity to the measles a larger percentage of them are about to recover from the disease than the Cayuse. Already suspicious of Whitman's motives, many tribe members start to believe that Whitman is purposely letting Cayuse die. At one point a wave of measles wipes out almost all of the Cayuse children.

The way the disease was playing out, it was unlikely that the Cayuse as a people would survive one or two more measles outbreaks, as almost all the children were dead and there weren't many adults remaining. In my opinion, the Whitman Massacre was the act of an insulted and desperate people who felt cornered and reacted in a way that was traditionally appropriate for their tribe.

You didn't ask about it, so I won't go into much detail about the resulting war, only that what little was left of the Cayuse did not do well. It is likely they expected the surrounding tribes (the Yakama and the Umatilla in particular) to side with them, but they did not get much support. Several members of the tribe gave themselves up to stop the war, and the surviving tribe members joined the Yakama, Umatilla or Nez Perce.

Much of this comes from a PNW History instructor from my undergrad, the research I did for my undergrad thesis on the Walla Walla treaty in 1855, a friend I have who works closely with the Nez Perce as a historian/curator, and a book I have kicking around from when I taught PNW History, though I have recently moved and don't have it handy. If you'd like I'll dig it up. Here are also a few websites that I directed my students at: The National Park Service Whitman Mission website A PBS site that is affiliated with the Ken Burns "The West" documentary, Narcissa Whitman's journal excerpts or the collection as a kindle edition for a buck.

If you want to read about a more successful Washington mission, I grew up just down the road from the Ahtanum Mission near Yakima. It was a Catholic mission instead of protestant, and the priests there ended up helping the Indians during a conflict which resulted in the mission being burned down. Here's a quick article about that http://www.historylink.org/index.cfm?DisplayPage=output.cfm&file_id=5285