... yeah, I always thought this ending was crappy. Okay, fine, the Ultramarine Corps kill, check. The Ultramarine Corps are mind controlled by evil pixies under Grodd into doing horrible things, check. These two assertions are somehow linked... how?
I mean, superheros get mind controlled all the time- including the JLA. Being puppeteered by 'mystery cosmic neuro parasites' is a perfectly valid defense in a world which has seen things like Starro multiple times. Being willing to use lethal force has no real bearing on this, unless they want to say that because the Ultramarines were willing to kill in the usual course of work, they were more susceptible to it/would kill as puppets. And if that's so... they did a really shitty job of conveying that sentiment, because you could easily have swapped the two teams in this story, sans this speech, and it would have been totally plausible.