In re the "heroes": they are young and inexperienced.
Way OP for their understanding of the world.
So, yes, doing dumb stuff is par for the course; they don't have the experience required to know any better.
In re the "bad guys": Battle Junkies doesn't begin to cover it... it does, actually. The real bad is the noble hiring them, as he does have an Evil Plan.
The two of them? No conscience, no remorse, except... they had no problems with letting the farmers and smugglers vacate the area prior to commencing hostilities; they were willing to wait until that had been accomplished.
So, while having no qualms about killing, they do want it to serve a purpose; backstabbing her party of adventurers during the subjugation served a purpose, it provided sustenance for the red sphere.
The farmers and smugglers perishing during their conflict would have no positive benefit for them, so why waste the lives that could be harvested later?
That would be the pragmatic evil interpretation, at least.
It's possible it was just that there would be no challenge involved, so it would be a waste of effort; that's the pure Battle Junkie interpretation.
Either way, it didn't appear to be a concern for them as individuals, but rather that killing them would be a waste of effort with no benefit justifying it; less effort was involved in waiting for them to clear the area than would have been involved in killing them.
The cost/benefit analysis said let them go.
There was nothing personal involved, it was just policy...