"If someone's taking care of a stray that would rather die, and they happen to kick it in anger once or twice, well, you have to think, that stray would be dead without them most likely." This statement I disagree with. Kicking your animal in anger not only can cause serious internal damage to the animal, but also shows that the person may have an underlying anger management issue. If a person kicks an animal in a rage, what's stopping them from kicking a young child that misbehaves?
I will admit that everyone's standards of pet care are very different. Not everyone can give the spacious home with interaction for many hours a day. Some people who truly love their pets have an outdoor/barn dog that isn't allowed in the house, but that animal is given some sort of shelter from the weather. (Side note: Dog houses can get super hot in summer, so make sure your pet has access to other shade and plenty of water if able.) Not every animal gets the best food, housing, etc. Some people call it abuse if the owner is stuck at work late, and can't let their pet out in a timely manner.
Now the tough thing about 'going to jail for killing' animals is that there are considered humane ways of killing animals, such as pharmacological euthanasia, exsanguination, or a CO2 chamber. Euthanizing a pet for medical reasons and preventing further suffering is in my opinion an ideal way to go about it. The euthanasia of an aggressive animal has always been a tough decision made often by both the owner and the veterinarian involved. From the veterinarian/animal care standpoint, aggressive animals are very tough to home, and may cause injury to a future owner. Shelters have been sued for adopting a dog that attacked a human after adoption. For this reason, aggressive animals are usually euthanized.
From the owner's standpoint, some people have animals that are very aggressive, and sincerely put in their best effort to give that animal a loving home, including seeing behaviorists. If that animal attacks a person though, often the police ask for the animal to be euthanized for safety reasons. Aggressive animals can cause serious damage to a person, and often there is very little that can be done to prevent a future attack.
Short story: A kitten was brought to the vet clinic I work at when she was about 3 months old. She was found, as well as other kittens and left at the clinic. The kittens were all treated for parasites and attempted to be socialized for re-homing. One of the kittens was very protective of the other kittens and would attack anyone that went into the cage. She bit, scratched, and spat at any person who attempted to enter, myself included. The other kittens were sweet and all moved on to loving homes, but the one aggressive black female cat was still there for four months. During that time, she was spayed and declawed on her front paws to prevent injury to anyone handling her. I wanted a pet, and no one wanted her, so I took her to school with me. She would attack me during the night for no reason. Sometimes she would attach with teeth and back claws to my leg or arm for no reason. If I tried to touch her, she would hiss and attack. I have scars on my left arm from her from over a year ago. If a person would have hit this point and chose to put this cat down, I would have accepted that they at least tried to give this cat a loving home... but she was my cat, and I'm a stubborn person sometimes, so I kept her. It's been almost two years since I brought home this little monster, and now she is a tame, somewhat friendly cat. She will never be a cuddly cat, and I'm okay with that. She hasn't attacked me in over a year now, and I love her a ton. Meet Ghost:
Why this cat was aggressive, I have no idea. She was a vicious little kitten that was found under a pool deck, no abusive history, but still the meanest kitten I have ever met in about 8 years of working with animals.
Just saying not all aggressive animals are abuse related, and not all aggressive animals will ever be 'normal'.
Now, this said, I am completely against animal abuse, but there are so many grey areas to make solid laws about anything. If killing was considered abuse, that may cause trouble down the line for veterinarians performing humane euthanasia. If laws protecting animal behavior issues existed, who would be willing to sacrifice their health for the hundreds/thousands of euthanasias due to behavioral problems? (Note: Before behavioral euthanasias are done, most veterinarians will be sure to rule out any curable medical condition that could be causing the behaviors.)
Sorry for mini story-time/rant day...