Warhammer 40K: Every Faction Ranked From Least To Most Evil

The Necrons were given a raw deal. Born on a homeworld bathed in the baleful radiation of a hostile star, their bodies were wracked with malignant cancers that rendered their lives both short and miserable.

Then they encountered the Old Ones, an advanced, near immortal species. This is where their problems began. Asked to share the secrets of their long life the Old Ones refused. Understandably the Necrons were aggrieved and responded by going to war, but cursed with their weak bodies they were easily defeated. Furious, the Necrons then turned to the Star Gods, beings of immortal energy, and in a twist of irony received the immortality and vitality they’d always wanted – by being turned into cyber skeletons and enslaved.

In time the Necrons were able to overcome the Star Gods and enslave them in turn but these experiences left the Necron bitter towards the living, and in a bout of sullen, resentful spite they have decided that all the other races must die.

One can sympathize but spite is a petty reason for galactic genocide, especially when you're self-righteous about it.