40k is about fighting fire with fire but where the burn never ends and all are eventually consumed. There is no path that doesn't lead into darkness, no hope, no mercy and every choice is merely deciding the lesser of two evils. It's a place where everyone points the finger to all others decrying them as the bad guy (and by most any standards of decency, being right since ultimately they're all the bad guys). A galaxy so riddled with darkness that by comparison a brutal dictator at the helm of trans-galactic war machine of an empire is considered by most to be the best worst option available.

What's the essence of 40k? Realizing you're part of a universe where "winning" is just a matter of surviving for as long as possible, one where all the odds are stacked against you yet you must learn to fight ever onward regardless of the hopelessness of the situation. Basically 40k is about not going gentle into the long night.

I agree with you on this as well.
When you look at the imperium and the emperor objectively you see a xenophobic and highly racist species.
Using brute power and singular willpower to subjugate all others they come across.
Humans have to adhere to a strict set of rules or suffer the consequences.

Non humans are just pure out of luck and are usually eradicated at first sight.
( Usually at least ) Some sub species like ratlings and the like are tolerated.

It is a universe were deviation is punished and blind obedience rewarded.
Sadly the whole idea of the emperor was a non religious empire. And now he is revered as the god emperor.
I think that the blind faith of millions have TURNED him into a godlike being since his essence is in the warp and the warp shapes things from emotions.
So at last humanity created their own god.

There is no black and white in this dark universe.
Not inherently bad nor good. Only lesser gradations of evil.

To me, 40k is the perfect representation of the potential universal fundamental "truth," of life that we know.

Everyone is looking out for themselves. The Eldar, the Imperium, the Necrontyr, Chaos, even Tau. All they're doing is trying to enforce their own agenda above everyone else's, using morality as an excuse to give their desires "logic."