Mormons run into the same problem with their own descriptive adjective when they get mad at splinter groups (including the polygamist churches) who call themselves Mormons. "We aren't those people! They cannot steal our identity! Blah, blah!" Historically, those groups have every bit as much right as the LDS to the adjective Mormon. When we get mad at them for using it (and daring to use it differently than we do), we only reveal our pettiness. (Is religion about words for us? Do we really care that much about adjectives, for Christ's sake? What is the New Testament really about, people?) The only moral position is to let our actions speak for themselves. If you want to get a message of goodness out into the world, you have to be good. You cannot waste time fighting about stuff that (1) doesn't really matter and (2) that you are never going to change by fighting. The fact of the matter is that historical Christianity has always given birth to heretics, much to the chagrin of the orthodox. Many Catholics would expunge the Protestant Reformation if they could. Many Protestants would expunge the Mormon Restoration. Many Mormons would expunge the schism that produced the FLDS. But history isn't about what we would do. It's about what other people already did. Historically speaking, Mormons (including the FLDS) are clearly a Christian offshoot, different from other offshoots but not categorically separate. (The Mormon vision of Jesus, particularly in the Book of Mormon, is recognizably Protestant, with a few tweaks that drive Nicene believers crazy, though I had a professor at BYU who showed us how Mormons could embrace the Nicene creed, if we were willing to get creative with the meaning of the deliberately vague Greek words used to craft it.)
Call us bad Christians, deviant Christians, heretical Christians, anti-Christ Christians, or whatever you want, really. It doesn't really matter, and it won't really change anything (except insofar as it contributes to emotional sectarian feeling on both sides). And that exclusivist streak that you find in us, that arrogance that presumes to judge other Christians and find them wanting? That is vintage historical Christianity: Joseph Smith took it from the Christian movements around him. (Read some of the proselytizing pamphlets from the era: slandering the other guy was the way to preach back then.) Not only that, it goes all the way back: as far back as we are aware of groups of people calling themselves Christians, we find them at one another's throats (literally or figuratively) over the fact that they cannot agree about stuff. (Read the New Testament, especially Acts. Notice Ananias, Sapphira, and the fight between Peter and Paul.) Christ came to bring a sword, didn't he? But it is ultimately unfair to make partisan craziness uniquely Christian: we find it all over human history, before, after, and outside of Christianity (as well as all through it). People separate into groups and fight about whose group is best. If we're lucky, we just call each other names. If we're not, we end up with wars. C'est la vie. I wish it weren't so. I used to think that Christians should be different. But historical research has entirely wilted my naive optimism.