fight fire with fire; set a thief to catch a thief
Proverb meaning to use the same harmful means to counter a problem, especially using an evil to defeat another evil.
As the saying goes, 'fight fire with fire'; sometimes it's necessary to use bad means to defeat evil.
Variant using the kanji 以て for もって; less common but still seen.
A Japanese form of the Chinese maxim 以毒攻毒, “use poison to attack poison,” expressing the idea of combating one harmful thing with another.