Compared with Dark Eldar, Chaos, and Necrons Orks look innocent (MY opinion).
Compared to Eldar, SMs, Tau and IG they are bloodthristy killers (again MY opinion).
I'd have to say Orks rank with Tyranids. They kill because they have too. Tyranids to live, Orks to live. When I say that I mean Orks aren't the kind to drop everything and flee from a large foe. They are the fearless nomads of space to put it simply, willing to take on anything.