An overkill occurs when a monster is killed by a critical hit. A red skull appears above the enemy's head for approximately seven seconds, during which the player can melee attack (default key: F) to deal a finishing blow to the enemy for double experience and extra (approximately triple) adrenaline.
During this time, the enemy also enters a death animation, the details of which depend on the damage type of the weapon that was used to deal the critical blow. For example, a physical weapon will make the enemy hunch over as if about to pass out, while an electrical weapon will cause electricity to arc out from their body. When correctly performed this will also deal additional damage to any remaining mobs close by with the damage type dealt to them corresponding to the damage type used to trigger the overkill.
If the monster isn't killed during the overkill period, it will simply die as usual, granting normal experience and adrenaline. If this happens the mob can still be ressurected.