Read both of the Twice Dead King books as well as the Infinite and the Divine if you haven't.

It makes it pretty clear that they are insanely barbaric toward their own people even. Nevermind that they view all other life with disdain and straight up anger despite showing sentience.

Some details from the aforementioned books. It is made clear that the average necron warrior may infact retain sentience yet due to the silent kings decisions they cannot even really control their own body. Forced to stand sentry duty, motionless, for centuries, all the while their mind is active. They are enslaved to the will of the necron nobility basically. Only the nobility and those who serve some sort of important role were allowed actual freedom. The vast majority were not.

Also in Twice Dead King there is a flashback to when they were the Necrontyr, the primary character who is a Prince goes with his elder brother to watch soldiers training. They watch and bet on which will win their mock duels, having a grand old time. Then afterwards the elder just executes I think it was 50 of the soldiers, ones they were watching. He did this just to prove a point to the younger brother, some shit about how the Necron dynasties are a fire and the necrontyr are the logs. You don't weep for logs when you cast them into the fire. Nobody bats an eyelid at the completely random execution of 50 soldiers. Nobody gives a shit. The nobility can just do that.

There is plenty of other shit in the books that shows you how they really really are absolutely not in any measurable way "good guys". That anyone tries to argue this is laughable. The memeing about them just being grumpy old guys who want everyone off their lawn is not even close to accurate. Some people either don't do sufficient reading before deciding whats good and bad or they cannot separate memes from actual lore.

Don't get me wrong, I love the Necrons. They are awful however. An abominable society and culture both before they were robots and after. Everything is shitty in 40k people need to stop looking for good guys. If you want a good guy to cheer for you are in the wrong universe.