Bloodlust is one of Fallout GURPS disadvantages. See: Fallout GURPS.
A character with this disadvantage suffers from a strong desire to see his foes dead. He must go for killing blows in a fight, put in an extra shot to make sure of a downed foe, choose violent and messy options when stealth might be better, etc. An IQ roll is necessary to accept a surrender, or even take a prisoner under orders. And a player who always tries to make this IQ roll may be judged guilty of bad roleplaying!”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Bloodlust was recycled, image used for the Berserker reputation.