[@Letter Bee]
The Emperor fought against Tyrants, declared all men and women equal, and enforced a fair and just set of laws in a postapocalyptic world. He was also a highly scientific man, abolishing things like the religious bias against homosexuality.

The Imperial Truth was important, because unlike with real-life concepts, deviating from the Imperial Truth literally creates space monsters. It's partially because of the general populace's deviation from the Imperial Truth that the Chaos Gods have the power they do.

The destruction of the Diasporex and Interex was a result of the extremist views being utilized by the Primarchs who would eventually fall to the side of Chaos. The Emperor was opposed to alien life, but he wished to control it instead of annihilating it.

The Imperium of Man under the Emperor's rule was a good place, it was only after his fall that it fell into a state of zealous extremism and constant warfare.