Well lets have a rundown of the 40K factions(I'll try to keep this unbiased):
Imperium of Man-Space Marines are brutal psychopaths, Imperial Guardsmen are cannon fodder, Inquisition is just a bunch of pricks.
Eldar- A bunch of fruit-loops who spawned a Chaos god. Who then proceeded to rape their souls.
Dark Eldar- Sadists that torture a bunch of people cause they want to. Also, they're "awfwaad of Swaneesh," so they brutally steal other people's souls. The pussies.
Orks- Cool bros, live to fight. And totally not religious like dem Space Marines.
Necrons- Machines that only exist to kill every-fucking-thing. No real purpose. They're also ugly mother fuckers.
Tau- They're like door-to-door Jehovah Witnesses. Only on a galaxy wide scale, and with billions and billions of members and deadly fire weapons.
Tyranids- The coolest and undeniably the strongest race in 40K. They have a single goal in minde, and its to have everyone join them in their gene pools in order to evolve into the single most superior sentient being in all the universe(with long flowing blonde hair and washboard abs, like mine). Also, they have the fucking Swarmlord.
Chaos- Hipster Space Marines who thought the Imperium was too "mainstream", so they left.