On the necessity of killing:
I am repulsed by the idea of killing someone, but I certainly come down on the side of necessity. I would certainly pull the trigger on one man to save a thousand.
If that one man was trying to kill someone else I'd pull the trigger to save one. Provided there was no other option.
However, I think the comment about civilians being collateral damage as acceptable was directed more at the idea of once you are already at war. I doubt the tevs were just nuking shopping malls and apartment buildings. It's not their style. Tev assaults may be brutal and ruthless, but they are laser focused on their objectives. Not only would it serve no purpose to nuke civilian centres for the lulz, it would enrage the UEF population for no reason, and it would waste ammunition.
Steele doesn't do stupid. Odds are there were munitions depots, marine rax's, production facilities or some other economic linchpin important to the UEF war effort on Luna, which apparently got bombed. Especially since it was during the blitz. In fact I can imagine no other way. What kind of mission brief would Steele give?
"Alright people! We're going to take out Rheza Station, the Saab Shipyard, Hydura Station - and hey. We're gonna blow up some hospitals and schools."
"About that last objective sir. Why?"
"Because I like to hear the sounds of screaming civilians! Mwahahaha forward for our evil Tev overlords!"
Yeah I don't buy it.