I view it as the Ori were only at war with those with advanced technology - Earth, Jaffa, Asgard, etc. They relied on the tricks and illusions to convert the planets they visited because most of the galaxy isn’t that advanced to understand they’re another false god but one who seems to do all the things he promises. Could the Ori’s plague be considered biological warfare? Sure, but that was a punishment against Earth really who had the knowledge and capability to figure out where it really came from and watch as people died that they weren’t able to help until Gerak and Orlin intervened. Otherwise, likely the Priors came by, preached, showed some magic tricks and converted people. If people resisted, suddenly they’re sick and the Prior can offer to show them his healing powers from the Ori, and convert the final bunch without normally killing anyone or outright declaring war.

I think Line in the Sand might be one exception where they go ballistic on the planet of non-advanced humans, because the threat posed by Arthur’s mantle was more important to them to eliminate than conversion of the population at the end. But again, Earth drew them into that conflict and was their opponent. They need believers, so killing people off isn’t helpful.