Although I do not want to compare Humans with Animals. If a human wants to eat a human being and kills one, why is he/she punished? Because it is against the law. Simple, if you kill cow in Nepal, it is against the law and punished.
"In Paris, act like a french" bhanne ta sune kai hola. Similarly, the country which values the cow and prohibits one from slaughtering the cow, you should not do it. It is as simple as this.