Why kill someone because he has killed a cow, that's very stupid, if the cos is a god why doesn't it avenge itself from being slaughtered, whatever god tells another to avenge it is a toothless god.
just like OP said that actually this is teachings of certain religious heritage that they consider the cow as holly animal and for those who kill them the people will believe it's just like killing human however in India there is an extreme actions that they had drink cow urine because they believe Will bring good luck for them