Most people are at the bottom of the hierarchy. Even the Imperial Guard are among the top-tier forces of the Imperium. They have better equipment and training than PDFs (planetary defense force) or the private mercenary armies of whatever rich fuck. And being in the Guard still sucks.

The lower castes within the Imperium have few (if any) rights. Some are slaves doing horrific labor in even worse conditions. Some are turned into servitors, performing the tasks of AI and computers without being AI. Some are harvested for organs or experimented on. Some are straight-up killed by the wealthy for sport. And this doesn't even get into what kinds of things are done to humans by various aliens and chaos worshippers.

It is the cruelest and bloodiest regime imaginable. An eternity of carnage and slaughter, and the laughter of thirsting gods.