Xenomorphs, the Tyranids and Dark Eldar.
Xenomorphs use you as either food or incubation chambers.
Tyranids use you as food and your DNA to mutate more of themselves. They do not stop until nothing but them is left alive in the galaxy. Then they travel to the next galaxy and repeat.
Dark Eldar: While they do not use you as food (some of the time at least), they are sadistical and not only have a liking but a real talent of inflicting pain coupled with several millenia worth of experience in torture and dealing out painful and prolonged deaths. Oh, and they might brainwash you while they are at it, allowing you to live in perpetual agony while you serve them. And during the entire process, from capture to eventual end, they will laugh at you, genuinely enjoying every little scream and whimper you make.
And let's be honest: any one of those races could screw us six ways to sunday.