Sorry if this is too crazy or off topic since you did say "animals" in the thread title, so I hope you can just ignore it if so, but I thought of it again as I read these responses. Why are we usually more concerned about killing animals, compared to, say, vegetables and viruses? I guess if something attacks you it's easier to justify killing as defense (i.e., viruses).
I've occasionally wondered, and argued with vegetarian/vegan friends, if we have less "respect" for vegetation than we do for animals simply due to human conceit that makes us think we're the most important living things in the universe, or maybe due to a lack of understanding of the equally important place vegetation has in our universe compared to animals (including humans). When I read about new discoveries about ways that trees and other vegetation "communicate" it just gives me more food for thought
...mental omnivour