Opinion: Common sense and "rational and prudent person" is how I judge. Next is circumstance. I myself might be in less danger than a small child or an elderly woman, so third-party defense can be quite rational.
IMO, defense of my property, such as my pet in my yard under attack by any trespassing animal, is quite justified.
The mere sight of an animal in my very-rural yard, however, does not alarm me. I have no pets. I have coyotes, foxes and bobcats traipsing through from time to time. Mostly, I take pictures. I'd not be surprised at a cougar coming through at night. No harm, no foul. A stray but apparently not-feral dog is annoying, but not harmful.
You're from BATFE? Come right in! I use all your fine products!