"Do unto others as you would by done by" is fine as a basic principle the problem comes when you're dealing with some one whose motto is "do unto others what ever you feel like". In this situation unless you rapidly adapt you're going to get trashed.
I think Idris Shar said it well "Only a fool is honest with the dishonest"
If your walking down the street and some one pulls a knife on you, you're under no obligation to be nice to them.
All the best PP