Necrons aren't necessarily 'evil', but they're not nice either. They're the old man who yells at children to get off his lawn...except he hasn't lived in that house for fifty years, someone else is living there now, and he's got an assault rifle. Their worldview starts at reclaiming the empire they lost after millions of years of hibernation, and ends at making that empire even bigger than it was - having given up their life energies/souls to the C'tan when they were transformed, they're forever frozen developmentally at that point in their history. The living races look to the future, but all the Necrons can do is try and rebuild the past, and in the process, they simply don't care who or what they trample over on the road to that old glory. So they've got some pathos and tragedy of their own.
Craftworld Eldar aren't exactly evil, but they are possibly the most utterly arrogant creatures in existence. Their entire culture and mindset revolves around the 'undeniable' truth that the Eldar are the greatest things to ever walk the stars, even with how far they've fallen since their prime, and no non-Eldar is fit even to be used as the ground they walk on to keep their boots from getting muddy. They might hide this attitude for diplomatic reasons when they need to, but the idea that a planet full of humans dying now is a perfectly acceptable sacrifice to save the life of one Eldar who would otherwise die later (which is something they have literally done, via the divinations of the Farseers) is how every last one of them thinks at the core.
Oh, I do enjoy them... although it kind of worries me
Although I'm not sure if a sympathetic DE character is necessarily emo... I can imagine that it could be interesting to explore someone who is a product of this vile society, but starts questioning it for some reason.
But okay, I admit that I have a knack for sympathetic monsters and scary people seeking redemption. I've been coming up with such characters for years...
You've probably been told this before, but don't be worried. Like I said, there's a little monster inside everyone - the good people just keep it little and quiet most of the time, the bad ones let it grow big and loud.
And as for a sympathetic DE character...maybe not emo, but definitely tragic. You could go that route, but it would be a tragedy in the classical sense, because it's not just a cultural and societal tendency towards vile sadism. They're literally physiologically dependent on it and die without it - and if they die, their soul is snack food for a Chaos God, so there's not even any sort of 'redemption through death' waiting at the end.