@ysmina-- Yes, but that's kind of the point. It doesn't say "all is fair" for no reason. It means that you may do anything because in certain situations, you are not bound by the regular rules. The important thing is to win. How you win is not important.
Yea, "all is fair in war and love" has been around for a very long time. As far as I know, it has been traced all the way back to Greek literature. It was first used in English by a British author. But the first American author to use it in American literature was John Pendleton Kennedy. I think the literary piece he used this phrase in was called "Horse-Shoe Robinson" or something similar to that.