Myself, I only recognize cats and dogs as legitimate "pets", others are just critters to me that essentially I just simply do not care about. So killing them for me is no big deal whatsoever, keep in mind I'm not a hunter, just not sympathetic to any animals, or bugs, cause. For me not to kill an animal that is dangerous or a nuisance to me, it must serve a purpose which I benefit from.
I've certainly killed my share of mice for coming into my house, I do not care if they knew what they were doing was wrong or not, but they serve no purpose to me except to be pests, spread disease, and attract snakes.
I've also killed some barn-swallows who made a nest over my porch, didn't really want to since they are great mosquito killers, but their crap was ruining my porch and their incessant chirping was infuriating.
I will kill a snake, but I try not to as they do keep the mice at bay. I pretty much let all non-poisonous snakes be, I just wrangle them a chunk them over my back fence, which butts up to some farm land. I will certainly take out a poisonous snake though. With my young'un running around, I just cannot risk a second encounter with any Copperheads or Rattlers.
I've also taken out my share of possums, armadillos, coyotes, etc. etc. Now, I don't mind these animals as they do have a very important purpose in our local food chain, but when they start coming close to houses and civilization there is something definitely wrong. The possums and armadillos usually keep at bay, so if they start coming around either they're looking for easy food or rabid. So, if you don't have any easily accessible food outside, like cat or dog food, then there's a good chance they're rabid. Coyotes don't hardly ever come around at all, so if one starts it's just best to take them out or they'll take out your pets first. They love feasting on small dogs and cats. If you can shoot a couple a year, that keeps the rest of the packs away.
Most spiders can escape my wrath if they are outside, they work on my side by keeping other bugs in control. Once they come inside, they're more than likely a goner if I get my boot moving quick enough. I already have plenty outside, don't see the need to try to pick it up and put it outside.