An old series that I really enjoyed was Jack L Chalkers Quintara Marathon. It sounds corny on paper but is expertly handled and tons of characters die. Most in fact.
To break it down humanity simultaneously develops faster than light capabilities and various psychic powers (telepathy, fire starting, heart destroying, etc...) although most humans have none or at most one (one very strong power is a null...someone who can't be affected). Humanity starts spreading amongst the stars and for awhile it's all good, we are kings! But then when we get big enogh to be worthwhile, three separate and competing master races swoop in and take over our little federation. Each master race and it's subordinate lesser races basically now "own" a third of humanity. One master race is basically a planet sized super computer that apportions its resources for the most profit, basically a giant unfeeling corporation at its best/worst. The second master race is a race that has incredibly powerful psychic powers and always projects itself as the subordinates equivalent of "angels" but in reality are ugly lumps of winged flesh. The second race is basically the Catholic Church at its best/worst, a giant religious bureaucracy that is ostensibly benevolent to its lesser races and concerned about the welfare of the individual but demands worship and obedience. The third master race is an intelligent and telepathic virus that lives literally inside the lesser races as host bodies and the uninfected are viewed and treated simply as slaves and chattel to do the infecteds bidding. Basically the third master race is a mix of pre-Civil War slave plantations and mines mixed with the more horrifying possibility of the slaves losing their identity.
So these three master races live in a state of a "warm" Cold War where no full scale war is occurring but lots of competition to find additional resources throughout the Galaxy and low grade but fatal conflicts occur frequently in the rush to and to protect resources. Then from out of the blue, demons start appearing and annihilating worlds all throughout the known Galaxy taking out even members of the master races. It turns out that while every race has a different and even frequently multiple ideas about what good gods and angels should look like, ever race across the galaxy has a shared and almost genetically encoded idea as to what evil or demons look like and it's the stereotypical goat legs, leathery wings, and horned headed devil. In order to stop the demons, the three races send their best multi-race exploration ships on an expedition to explore the interior of a tesseract that appears to be the center of where demons appear. Each of those explorer ships have at least one human on their main exploration team.
This sounds like I gave the whole story, but that's really just the first maybe third of one book that deals with lots of deep ideas on humanity, godlike powers, social structures and more all while in books covered in action with the characters blood a mile deep.
