What's the difference between animals and people? There are no monsters in the real world, not truly, but they abound in our fantasy and sci-fi games.
Monsters are not people, they are aberrations, corruptions, dangerous beasts that not only can be killed but *should* be killed by heroes protecting civilization.
People are sentient beings just like you and me, but might have fur, or horns, or a tail. They can certainly commit evil acts, or just be violently opposed to other races . . . but wantonly killing, maiming, or violating them without regard is evil, just as it would be in the real world.
The difference isn't always easy to judge. Are tieflings people? Are actual devils from the pits of hell people?
If you and I, both human, were enemies, and I captured you . . . would it be okay for me to strip you naked and shave off all of your hair forcefully to prevent you from hiding dangerous weapons? Would it be okay for me to break your fingers to prevent you from wielding a sword or casting a spell against me?
Is it okay for me to use a riding crop or whip on a horse, cattle, or other beast of burden? I can certainly be cruel about it. Is it okay for me to use a riding crop or whip against a person (in a non-combat or kinky BDSM situation)?
Dehorning cattle, or a rhino (we do this to protect rhinos from poachers), is not an evil, inhumane attack. Dehorning a monster isn't necessarily evil either, and might be a necessary thing to do. Dehorning a person, even a 9-ft tall person with sharp horns and hooves, is an evil act upon that person. Certainly, not all evil acts have an equal impact on the victim. I'd rather have my property stolen than be raped or murdered, I'd rather be raped than murdered due to the permanence of death. But do we really need to compare horrific acts?