What about all's fair in love and war? That is, things that might normally be considered unacceptable are/can be used.
From Merriam-Webster:
—used to describe a situation in which people do not follow the usual rules of behavior and do things that are normally considered unfair
Sure, it was underhanded to steal his customers, but all's fair in love and war.
A discussion of the phrase's origins can be found here.
