bloodsucker (n.)

• barnacle
• bedbug
• blackmailer
• extortioner
• extortionist
• freeloader
• hanger-on
• harpy
• leech
• moocher
• mosquito
• parasite
• predator
• profiteer
• racketeer
• raptor
• scrounger
• shark
• spiv
• sponge
• sponger
• sucker
• tick
• vampire
• vulture