Sink to somebody's level - Idioms by The Free Dictionary
sink to somebody's level
sink to (someone's) level
To behave as badly, immorally, or basely as someone else, especially in a dispute or conflict. Don't hit him, Tom, just walk away; don't sink to his level. We'll have to sink to their level if we're going to stay competitive in the market.
sink to somebody’s ˈlevel stop behaving well and begin to behave badly, especially in an argument or a fight, because other people are behaving in this way: Use words, not violence, or you’ll just be sinking to their level.