I don't think you're entirely wrong there, although I would probably still label the IoM pretty damn evil. Yes there's alot done for survival and that is what it is but... there's also alot that's not. When the IoM is willing to genocide a planet for like, not properly worshipping the Emperor... that's about as evil as it gets.
I would disagree with that.
Consider the danger of not properly worshipping the Emperor: Warp entities gain power from emotions, including beliefs. Even if the Emperor is not a god just yet, he is well on the way to becoming one because he is being worshipped.
Improper worship however risks giving rise to a hostile Warp entity or alternatively empowering one of Chaos gods or Daemons.
So really, genociding a planet for not properly worshipping the Emperor is not really "as evil as it gets". Especially since Imperium only does it
in extremis. You see, Imperium is in fact willing to accept certain heterogeneity of beliefs - it actually practices syncretism on a galactic scale. You can worship Jesus, Greek Pantheon, Buddha, spirits of nature... so long as you accept the Emperor as God or at least "God in Chief" (e.g. Zeus for my Greek Pantheon example), Imperium doesn't really care about the specifics. So you have to be
really out of the line for Imperium to genocide you.
When looking at the overall picture, Imperium is probably
more tolerant than like 99% of monotheistic religions historically were.
I also don't think it's ENTIRELY true that some of those ideals don't work. The IoM could have... some peace and progress if it wanted to. The Tau would almost certainly be willing to come to the table and talk about not fighting... but, they're filthy xenos.
And Imperial attitude towards the filthy xenos is again, by and large, justified. Pre-Fall Humanity in fact was all too happy to cooperate with various xeno species, only for those to turn against humanity as soon as doing so became viable. So saying that Imperium would even have "some" peace and progress if it only made peace with xenos is not actually supported by the available evidence, and in any case, Imperium is not exactly in position to risk it.
Also, Imperium is
nowhere as extremist about "Kill All Xenos" as you seem to believe.
Imperium and Tau are in fact willing to cooperate against external threats. But Tau are still an expansionist power bent on galactic conquest - their ideology is basically Space Communism for crying out loud - so even entertaining idea of some grand alliance or permanent truce would be extremely dangerous.
I want the IoM to essentially be "bad guys". It's part of the fun.
Define "bad guys". Because when you look at it,
everybody is a "bad guy" by modern standards.
But Imperium is still one of the better factions. At least they and the Craftworld Eldar are fighting for survival.
Others?
Tau are Space Communists who want to establish some screwed-up 1984 / Brave New World crossover galaxy-wide.
Dark Eldar are psychotic BDSM fetishists who do
not ask for your consent before pulling you into bloody orgies.
Orks are psychotic football hooligans who want to do nothing but fight.
Chaos are psychotic cultists looking to feed the entire galaxy to their gods.
Tyranids are a swarm of space locusts who took a look at the galaxy and concluded "GIANT PIZZA SALAD BUFFET"
.
On the topic, a mild derail here with a personal thing i'd like to see and kind of headcanon into 40k... the Emperor's carcass on the Golden Throne isn't actually doing anything. The things the IoM attribute to him having a hand in is done through some ancient technology still up and running or what not. When Guilliman comes back an "communes" with the Emperor... he knows exactly what he's doing and what's up.
That explanation, I think, is completely unnecessary and runs counter to established canon for how beliefs and Warp work.
Even if you assume that the Emperor had been long dead, fact that citizens of Imperium
believe that he is alive means that yes, he is alive.
Also, we have Guilliman's inner thoughts confirming that yes, the Emperor is indeed alive.