It's not like calico2222 is asking whether she should have shot the cat. It's also not like she has any remorse for shooting it, because she's admittedly glad it's dead. (yes, I know, DH did the shooting)
Shooting an animal can be a first resort, or a last. It probably is the quickest, cheapest and definitely the most permanent course of action. It's just not a method that everyone resorts to. Maybe it's the title of the post itself...We had to kill a cat today. Whether they actually had to or not is questionable, at least to some.
I'm not sure anyone is arguing. If calico222 did not want to discuss it, what was the point of posting? Is everyone supposed to say, "Oh yeah, you did the right thing," whether they believe it or not. If you don't want to know what others think, why ask?
Whether the cat's behavior would have changed or not if neutered is only a matter of speculation, but that holds true for both opinions. No one can say with any certainty either way.
And saying that it's okay because he went to a better place, well...I'm not even going there.