Cultures and social norms develop over time. Whether it is slavery, women having the vote, or anything else, the fact that it was ever the norm or part of culture, is not a justification for it. If you think that culture is a justification, then if you look at other cultures, you must advocate every single practice that they do, regardless of how clearly unethical it is. That's not a rational point of view. You should be able to form a view on a practice regardless of where it happens. So if you say that killing dogs is unacceptable because your culture says so, but you think that it's fine if other cultures do it, consider the following: If someone is about to kill a dog in your culture, would you really say "Excuse me, can you please cross the border to that other culture where that kind of thing is the norm? Then I will stop caring about that dog". This is about the victim, it doesn't matter where it happens.