Yes, the imperium is portrayed as sympathetic.
Because the Imperium is made up of humans, it's human nature to sympathize with humans. For the record there are chaos gaunts ghosts.
They're called the blood pact of Khorne.
Yes, you see the imperium as being justified, because Chaos is represented as being that bad, what would you call a faction whose only desire is to destroy everything?
What would you call a faction that only cares about defeating the other three chaos gods so that they can reign supreme?
Chaos doesn't care about anything except its own power, yes Chaos offers freedom.
It's lies, when Angron became a Daemon prince did he become free?
Do these ones look like they're free?
Chaos lies and lures victims in and then twists them into something that is no longer themselves.
Horus fell to Chaos because Chaos told him that Emps planned on turning the Imperium into a grimdark hellhole where he was worshipped as a god and Himself and eight of his brothers would be forgotten.
Horus turned because he believed that it was true.