A bloodsucker was one who drank another's blood. Metaphorically, it could refer to someone who profited from another's acts.
In 2285, James T. Kirk called Khan Noonien Singh a bloodsucker after the Augment used a Ceti eel-manipulated Clark Terrell to try and kill him. (Star Trek II: The Wrath of Khan)
In 2375, Tom Paris referred to a spider he was trying to catch as a bloodsucker. (VOY: "Gravity")