kill em with kindness

Kill em with kindness is the phrase commonly used when you want to refer to the concept of being overly nice and kind to someone unpleasant or who is one of your enemies as a way to prevent further judgment or unkindness from them. You would usually kill someone with kindness when they are being very unpleasant to you and instead of acting the same way, you choose to be the better person by acting like a more nice and kind person to them.