All is fair in love and war.
I think people tend to overlook the true meaning of this phrase.
The common assumption is that people are not bound by rules in love and in war.
This is usually used as justification of another love interest or third party.
However, if one interprets this phrase in a Nietzschean point of view, one comes to this conclusion: The rules that bound fair play in love and war are grown from the same soil in which we bury our enemies.
We punish people and kill people all in the name of fair play, thus fair play becomes escaping punishment! The one who evades punishment, lives in good faith to fair play.
But this evasion is simply that, an evasion, a ruse, a guise, to hide ourselves from ourselves.