All's fair in love and war

This must have been an excuse for many a wicked deed. People in love can verge on the irrational and might do things they would never ordinarily contemplate. Doing something unethical could of course rebound on the doer if the loved one (or rival) found out and didn't like it. In war there are rules and conventions but if they are ignored and you are on the losing side you could pay dearly. Love and war are strongly competitive situations and this saying indicates the intensity of an involvement that brings out the best and the worst of those concerned.