Some people just seem “immune” to the idea of empathy. I’ve heard pundits half-jokingly talk about the “liberal brain virus”, but sometimes I wonder if it’s actually the other way around: a real mental disorder that makes it difficult or impossible to imagine the perspective of others.
Of course, that’s just a guess based on absolutely nothing. It’s much more likely that some people are just selfish, insensitive pricks. They don’t get to escape responsibility for their behavior.
Do unto others as they would have done unto them. Takes the ethnocemtrism, religious centristism etc. out. Very easy to be very offensive/demaeaning by treating someone the way you want to be treated.
Step back a bit and be less specific, and it works better.
For example “I’d like to be greeted with a warm handshake and a friendly hello, so that’s how I’ll treat everyone” isn’t appropriate for all cultures. The general concepts of respect and courtesy, however, are universal. That’s how I’ve always interpreted “do unto others”.