"Does anybody know what we really have in our US backcountry waters that needs killing? Is there a source for this info?"
The EPA has a lot of information, but it may not be easily accessible.
The problem is that there is huge variation in water quality from one place to another. High elevation snowmelt in California might get you one thing, and low elevation canoe country in Minnesota might get you something totally different. In a mining state, you might be concerned about heavy metals in the water. Near a very busy backpacking trail, you might be concerned about e. coli.
The only rule is that there are no rules.
Personally, I tend to be operating in California at moderate to high elevation. Most of the time, I just filter with a gravity-fed Frontier Pro and that's all. If I am concerned, I also have some liquid bleach along. In some cases, I have been known to drink water untreated if I can see it melting directly off a snowfield.