To the original question "Why is Chaos presented as the more reasonable side". I'd like to answer with a short clip from a movie. It's an excellent scene where the Devil actually explains his actions logically so he seems like the good guy next to an uncaring/mad god.
From Devil's Advocate. Al Pacinos speech:
http://www.youtube.com/watch?v=RGR4SFOimlk
"The Devil: Let me give you a little inside information about God. God likes to watch. He's a prankster. Think about it. He gives man instincts. He gives you this extraordinary gift, and then what does He do, I swear for His own amusement, his own private, cosmic gag reel, He sets the rules in opposition. It's the goof of all time. Look but don't touch. Touch, but don't taste. Taste, don't swallow. Ahaha. And while you're jumpin' from one foot to the next, what is he doing? He's laughin' His sick, ****in' ass off! He's a tight-ass! He's a SADIST! He's an absentee landlord! Worship that? NEVER! "
"The Devil: You sharpen the human appetite to the point where it can split atoms with its desire; you build egos the size of cathedrals; fiber-optically connect the world to every eager impulse; grease even the dullest dreams with these dollar-green, gold-plated fantasies, until every human becomes an aspiring emperor, becomes his own God... and where can you go from there?"
""Better to reign in Hell than serve in Heaven", is that it?"
"The Devil: Why not? I'm here on the ground with my nose in it since the whole thing began. I've nurtured every sensation man's been inspired to have. I cared about what he wanted and I never judged him. Why? Because I never rejected him, in spite of all his imperfections. I'm a fan of man. I'm a humanist. Maybe the last humanist."
Switch "Devil" for "Chaos" and "God" for "Emperor" and it's propably the same speech that Chaos gods gave to the Primarchs that got them.

They made Emperor look like he was denying the Primarches' own wishes and holding back their personal visions for humanity. They promised things the Emperor had denied or forbidden. And they didn't judge if one Primarch was religious (Lorgar) or other interested in forbidden sorcery (Magnus), they embraced such things.