Insanity Strike is a vault 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 A Starry Eyed Encounter and Sly Cooper can use it; in Sly 3, it is purchasable for 600 coins during the first day of Flight of Fancy for Bentley.
Sly will tap his cane on the ground and Bentley will activate 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, they will instead turn on the user. If an unaffected enemy is attacked by an affected enemy, they will gain insanity. The effect wears off after a short amount of time.