Was it dishonorable to take advantage and kill the enemy while they're weakened?
It may be so, but they still went ahead and did it.
See, why fight a losing battle? Humans are already painfully aware that they're completely outclassed in terms of prosperity. Both the Celestials and the Hypogeans are old existences. Their foundations run deeper than humans could ever imagine.
Humanity was still in the process of catching up, and it will take a very long time for them to stand at the same level as the invaders. Unfortunately, they need to deal with them now so they could only resort to underhanded tactics to cinch victory.
Well, other people would call this underhanded, Ashton on the other hand calls it being smart.
Why send people to their deaths? They have more value in being alive than dead, so why not deliberately stage everything to sharply reduce the casualties?