Insanity Strike is a safe Power-up in Sly 2: Band of Thieves and a buy-able power-up in Sly 3: Honor Among Thieves. In Sly 2, it is obtained from the safe in Episode 2: A Starry Eyed Encounter and Sly Cooper can use it; in Sly 3, it is purchasable during the first day of Episode 3: Flight of Fancy and Bentley can use it.
Sly will tap his cane on the ground and Bentley will set his wheelchair, which will imbue their weapons with a purple aura. Hitting a guard with the aura will give them insanity, causing them to attack other enemies. If no other enemies are in the immediate vicinity, then they will turn on Sly or Bentley. If an unaffected enemy is attack by an affected enemy, they will turn and attack the affected enemy until the affected enemy is dead. After that they will kill all other enemies in the area.