I am not sure that everything is fair in love and war. The ideas of "cheating on someone", and "banning clorine gas" might be taken as the counterexample.
Certainly I would avoid a known backstabber in business to the point of not doing business with them, even if they had the better financial terms.
> Banning clorine gas was a silly thing to do -- it didn't exactly stop the next war and only made the concept of war less terrifying.
This is such a silly statement I don't know where to begin. They knew there was going to be another war, did they? How did it make the concept of was less terrifying? WW1 was the "Great War" because it was the first time war was terrifying for everyone (except the stupid British generals, but don't get me started).
On this statement, one would disagree with the nuclear non-profileration treaty and think that WMDs are ok because war would be more terrifying.
Certainly I would avoid a known backstabber in business to the point of not doing business with them, even if they had the better financial terms.