I'd never heard of this before. Interesting story after reading up on the massacre. The Mormons involved obviously tried to hide their involvement, so it's hard to pass this off as simply self defense of their religious freedom. It turned out to be a pretty shameful act on their part. Seems like they were driven to hysteria though by the threat of invasion from the federal government.
Nothing I read really seemed to point to much in the way of religious extremism other than what they seemed to think to be the defense of their way of life. It doesn't sound like their was any specific religious motivation to kill the infidels or anything like that (certainly not defending Mormonism either though).
I too am getting sick of the popularization of the notion that religion leads to violence and hatred, etc., whereas reason leads to tolerance, peace, good will towards men, etc.