You'll find that, when dealing with xenos, they tend to have radically different priorities and ideas about how the world should work than you do, so they'll often do things that look tranparently insane because it makes perfect sense in thier own mindset.<br/><br/>That's the main reason why the Imperium of man believes the xeno should be purged.<br/><br/>One horrid possibility is that the Green Sun Princes might just be trying to make friends with you, and blowing up your enemies, shooting at you, and then calling you slaves is their idea of a friendly handshake.  (For one thing, they DID seem to be using low quality machine translation for their transmissions)  On the other hand, they could also be insultingly incompetent attackers as well.