Yeah I'd seriously suspect their motives. Was it their cat that got tortured, did they just witness it immediately before? You know, all the questions you'd want answered about someone who just, say, discovered their spouse in bed with a lover. That would be some serious impulse control issues.
If it was premeditated, and they sought out and killed this person after having decided it was a capital offense to torture a cat, I'd want some damn good answers. Even if they gave a spiel about animal torturers growing up to be sociopaths or something, there could still be extenuating circumstances, and I'd probably end up reporting them to the cops after safely removing myself from the situation.
Premeditated. When that happens, it's wrong on almost all levels. I may like to watch Dexter, doesn't mean I approve of taking rights in one's own hand. Not when it is because a pet or a loved one has been killed and tortured either.
What people do on impulse, when seeing the torture happen, that is something different. But, wouldn't it be better if they went for saving the assaulted one? If, in that action you kill the perpetrator, I still think it needs to be investigated. Leniency would be appropriate in the verdict, but still, it needs to happen.
Hypothetical, if I killed someone having a torturing go at my kids, protecting them thus, or failing in protecting them. I would want to be questioned and on trial. I would owe it to my kids. They deserve more than a Mum in prison for what has been done to them. The justice would be in making clear what the perpetrator had done.
And what if you have it wrong when you think you kill animal abuser?
Scenario;
You see someone torturing a cat, noose around it's neck, doing nasty things with it's legs. The one doing that runs away, because of you approaching. And you try to release the cat. With an intense look on your face, you try to undo the noose around it's neck. The cat resists as much as it can. It is an awful sight. Then, someone "brave" comes to the fore, with a big pole. He smacks you on the head, and, because you being unconscious isn't good enough for him, he smacks you till your skull is broken and your brain is into a pulp. Animal torturers deserve nothing better than that..... And, he did caught you in the act didn't he? Hands at a noose, and a cat in all states. He was right in killing you, wasn't he?
Side note; in a society where people want their meat as cheap as can be. There is torture and violation of animal rights happening in the meat industry.
If you seriously mean that people who torture animals deserve to die, you should track the meat you want to eat from birth to death, from the egg till the slaughterhouse. If you don't, you may very well be paying someone, who is torturing animals. The meat industry as it is nowadays needs mass culling now and then, and they are far from animal friendly. You have seen the images, some beasts just don't get killed right, and go to destruction half alive. There is transport from pigs all through Europe packed in trucks in summer, overheated standing in traffic jams, to make it possible that all Parma ham is coming from Parma indeed. That are the mild forms of animal pestering. It can be worse. Track your meat, if you are serious about this Soph. Make sure it dies at ease before getting slaughtered.
I will not turn a blind eye to someone killing someone else. I do turn a blind eye to some mishaps in animal industry, I buy my milk cheap. I buy free range eggs, but everyone knows that that is a farce. I do hate torture of animals, and I even feel kind of guilty when I hang sticky ribbons in summer, if there is an infestation of houseflies. And when I drown wasps in a lemonade trap.
And hamsters and instinct. An owner can have the instinct to try to protect loved pets. I have seen people react vehemently towards cats, because their birds were attacked. Those humans acted on instinct too. Instinct doesn't make everything right. There are plenty of people trying to get away with things, telling they can't help doing it, it is their instinct.