Background and personality.
The Necrons started off very much in the Terminator fashion. Unstopable metallic killing machines, they'd appear without warning, kill everyone (or as many as couldn't escape) and then vanish. No communications, to warnings, just merciless attacks. Fallen Necrons phased out, so they didn't even leave behind remains for study.
The new ones (and someone more familiar with them than I am can correct me if I'm wrong here) were a race of mortal creatures millions of years ago. They uploaded their personalities into robotic shells (or were tricked into it, believing it would grant them immortality?), then went into hibernation for ages due to reasons. Now they're waking back up, and finding all these new races all over the planets that made up their Empire. Break out the shootin' irons, there's varmints infesting our worlds.
I believe there are a number of personalities / noble houses / dynasties within the Necron Empire, so you now have the possibility of Necrons working against each other, and they have their reasons for wanting to kill everyone else off.