The mormons were present on the planet for six months before the 'bugs' attacked, this is unlikely as the bugs are shown to be particularly hostile and animalistic, they would've all died in the first week, probably the first day really. And yet we're expected to believe they lasted six months when trained soldiers can barely survive a night on their planets? Something doesn't add up!
In my experience Mormons are pretty nice to their new neighbors for like six months or so until they find out you won't convert.
Chances are the Bugs and Mormons had cordial relations until the Bugs made it clear they weren't interested in converting to Mormonism.
But Mormons gotta Mormon so they probably kept sending out missionaries or maybe even tried to do a Mountain Meadows Massacre thing and abduct some orphaned larva. We will never know. Regardless the Bugs responded with extermination.
Ultimately it was a disproportionate response on their part regardless of their legitimate but still loathsome bug grievances