Make an $200 upgrade that makes the human's skin green and the head swollen to the point they can't wear a helmet or a battlesuit, but the swollen head should have 1/2 of the helmen's protection. An alien that delivered a killing blow to a human who has this upgrade with a melee attack (bite, slash, pounce, trample, but not barb, lightning) should invariably inflate and explode regardless of class and remaining health. This is particluarly useful to solo tyrants as they don't have any ranged attacks.