be out for blood

To seek vengeance in an especially hostile or belligerent manner. This phrase is often used hyperbolically, although it can refer to physical violence. I'd avoid the boss if I were you—he's out for blood and screaming at everyone he sees. Some neighborhood kids beat me up last night, and now my brother is out for blood. Oh, Kelly's the one who lied and destroyed my reputation? Well, I am out for blood now!