Please note that Garr then aggressively orders Loew to "attack". His intent to spill the innocent blood of the colonists
regardless is quite apparent. Garr would've most likely made an example of them to prove his point of being a ruthless jackass. Besides, Garr is not supposed to be taken at his word since he already lied to Loew (via omission or straight to her face).
A military takeover of the facility shouldn't need an "attack" at all, they'd just have to walk in because the threat of attack should be enough to cow the colonists already. Indeed, killing any of the colonists would be detrimental since aren't they are supposedly there to run the refinery in the first place? Why are they depleting the available workforce in such a wasteful manner? Sure they may kill enough of them for them to stop resisting, but who's gonna run the refinery then?