Meat-eaters can kill other animals but not cows

Cows, however, are never meant to be killed or eaten by human beings. In every sastra, cow killing is vehemently condemned. Indeed, one who kills a cow must suffer for as many years as there are hairs on the body of a cow. Manu-samhita says, pravrttir esa bhutanam nivrttis tu maha-phala: we have many tendencies in this material world, but in human life one is meant to learn how to curb those tendencies. Those who desire to eat meat may satisfy the demands of their tongues by eating lower animals, but they should never kill cows, who are actually accepted as the mothers of human society because they supply milk. The sastra especially recommends, krsi-go-raksya: the vaisya section of humanity should arrange for the food of the entire society through agricultural activities and should give full protection to the cows, which are the most useful animals because they supply milk to human society.

Srimad Bhagavatam 6.4.9 purport