Every person has a quantum aspect.  A place and time in which they exist.  A soul if you like, though it doesn't matter really.  The simple fact remains that earlier in life this aspect is a longer than it is later on.  Coupled with conscious thought the aspect becomes a powerful quantum structure.  The technology to tap that structure underlies all of our civilization.  It is a sobering fact that each time we wish to bridge the gulf between stars a child, old enough to be aware and understand its place in the universe, must die.  You can quantify the value of life to the credit and at the age of eight it is at its peak.  All economies have always been fed by the blood of children.

The Drive Engineer's Primer 4013 edition.

How do you go darker than Warhammer 40000 one might ask?

Is that yours or from 40k canon or from 40k novels?

I already use something like that in the value of Psykers devoured by the Emperor, not just children, but I imagine many psi-snacks are kids.

It's mine.  There was a thread elsewhere looking for a new, dark sf setting.  And I thought about it a bit.  The thing is that people don't like it when you start riffing on a new setting when they're asking for something that already exists.

I was thinking that a conscious and rational society that murders children for economic gain is darker than a superstitious and irrational society that murders people of all ages to survive.  That the raising of children exclusively for the fueling of space ships is just a special kind of wrong.

Anyhow, it's just a thought that hit me.  Honestly, I like my science fiction rational and bright or at least grey with some highlights and shadows.

I suppose the next thing we would need is a truly horrific enemy.  Something worse than gods born of human evils.  Perhaps a race that has the technology to destroy stars or even a need to destroy stars.  That's why the universe is heavier than it should be.  All the stars they've killed.  They're doing it to hasten the heat death of the universe because they want to create a new one and implement some updates they thought up.  But there's two warring factions doing it, edition wars writ large.  One faction likes the old universe before this one and wants to bring it back and the other thinks a better, cleaner and more functional universe can be achieved.  Their technologies are powered by the quantum structure of the stars they kill.

Our heroes are pirates, human and robot outcasts, free thinking scientists and social reformers.  They can't ftl jump without the murder of sentient life forms so they try to steal stored quantum structures from the ships they raid so that they, don't contribute to the crime at least.  Sure if you want to go really dark, at some point they'll have to kill some children or not prevent some greater tragedy, but let's figure that there are factories and refineries that produce fuel cells.  Children can be a hassle when you're trying to make a profit.  But let's be clear, our heroes are just a different stripe of evil.  The existing alliance is pragmatic and heartless but our heroes should be as bad in their own right, willing to make sacrifices to win or genuinely indifferent to the broader consequences of their actions.

I could see alien races being a problem with the setting just insofar as pushing a tentacled blob into the quantum reactor is less sympathic, even though it should work given that they're really people too. Ultimately I imagine all the races would be enslaving each other for less depressing (if not less morally questionable) fuel.

Well, there used to be a few morally and intellectually superior races but they underestimated the base nature of humanity and got used for fuel because they remained innocent for a longer period of time.  Inevitably they went extinct.  Oh there were bleeding hearts and traditionalists who wanted to save a few in zoos or felt that only human children were the only truly acceptable fuel.  There were also economic implications that depressed the innocence commodity markets.  But like any other renewable resource they were easily exhausted by a boom market.

There are also species like the Drell who are born corrupt and incapable of ethical decision making.  These have thrived in the stellar age, many having found positions in management, law, and government.