I just consider it to be a point of view. The Imperium will emphasize on the negative aspects of Khorne, while a follower of chaos will see all those aspects like martial honour etc etc. If you're playing Black Crusade, you read about the negative side of the imperium as well. In the eyes of a follower of chaos, the imperium are the bad guys. It works in 40k because there is no true good or evil, it's just all grey zone.

So basically, both sides are right.

If you're playing Black Crusade, you read about the negative side of the imperium as well. In the eyes of a follower of chaos, the imperium are the bad guys.

Indeed. You can do a very good Black Crusade campaign whose plot is essentially Star Wars - with the Imperium playing the part of 'Evil Galactic Empire'.

And I've never understood why game developers and story writers change canon.

The reason that the fluff changed that "fast" in 40k is that there is no canon. Each writers and game developpers bring the universe in different facets that reflects his preferences, hence the universe changing from a person to an other. The same is true for my perception and yours.

Ask Lynata and I, or Dan Abnett and Gav Thorpe.

It's also probably worth pointing out that we're talking about a setting that spans not just a single country or continent (as many fantasy settings limit themselves to), a single world (as other fantasy and a lot of sci-fi does), or even one solar system, but an entire galaxy...a galaxy that has history and has large swathes of places that have either not been explored at all by humanity or have been explored, but lost. That opens up an unimaginably vast amount of interpretation.

- How does the Imperium view the galaxy? How do Orks? How do the Eldar, the Tyranid, the Tau and so on?

Compare this statement to the following;

- How do the English view the world? How do Americans? How do the Chinese? How do Maori, Indians, Russians and so on?*

Even on our own planet, world views, opinions, religions and philosophies differ. Sometimes wildly. Now expand this to not just a bunch of planets, but a galaxy worth. That's several orders of magnitude more opinions, religions and philosophies. There's plenty of room for someone to write pretty much whatever they want about the setting!

*Please try not to attach any meaning or context to the order of nations I list here compared to the races I listed in the previous example; they're just the first countries that popped into my head as I was writing.