Context:
The Necrontyr grew up on a radiation-blasted hell-hole with pitifully short lives, and were driven to advance so they could be remembered after their deaths.
Finally after reaching the stars without warp travel, the aliment of the radiation would follow them and their children for eternity. they met the old ones and the Eldar, virtually immortal races living in the lap of luxury. Who already occupied all the habitable worlds forcing them to colonize barren rocks.
They also pleaded with the Old ones to cure them of their sickness, but the Old ones flat out REFUSED them, and said that "All life is sacred" and didn't want to modify the Necrontyr.
The Necrontyr waged war against them, lost, and were pushed back to their original radiation blasted system and quarentined their to die meaningless and painful deaths, all the while the Old Ones refusing to kill them because "hur dur all life is sacred"
Necrons found C'tan, made a deal with C'tan, tricked, now souless zombies aside from their lords.
War in heaven, wipe out most the galaxy, Old ones engineer the Eldar and Orks to fight the Necrons.
Old One warp weapons and the fighting cause the emergence of daemons.
C'tan successfully drive the old ones to extinction, before starting to eat each other at which point hte Necrons shattered them in a rebellion and now we have pokemon C'tan.
Sarekh orders necrons to go to sleep and burns out command chip, now Eldar are the dominant thing in the galaxy.
>old ones probably built the pharos.
So the Old ones literally are responsible for the Orks, Daemons, Eldar>slannesh>eye of terror, Necrons, and possibly even tyranids.
All because they refused the Necrontyr healthcare.