we often say to spay or neuter one's pets because to not do so would cause overpopulation, would continue the current overpopulation. following the same logic, we must euthanasize animals like dogs and cats, because there's an over population of them. there'd be too many, would spread disease, and cause destruction, and be in start poverty much more. animals are not like humans with an inherent dignity that means we can't kill them. killing people cannot be used as a means to an end. they are just animals and killing them is a means to our end.