All’s fair in love and war
Definition 1 of 1
Pro Tip 1/3Context Matters
Used in situations when someone does something unexpected or mildly inappropriate for a passionate purpose. 
When she hid his phone so he wouldn't call another girl, it was like all's fair in love and war.
Pro Tip 2/3Not for Serious Wrongdoings
Doesn't justify harmful or illegal acts; context is key for this phrase's acceptability. 
She felt bad about eavesdropping, yet all's fair in love and war.
Pro Tip 3/3Figurative Use
Often used figuratively to speak about competitions or rivalries in other areas of life. 
They spammed social media for votes in the school election - all's fair in love and war, right?