I've got a few ideas for antagonistic elves...
One Bad Penny
In my
Dungeon Damage world, the elves took the place of northern Europeans. And there are no shortage of evil folks from that region - just think like humans. An NPC I never really got to use was Bloody Johan, an elven mercenary from Bavaria. He wasn't old enough for the Sack of Rome, but he served in two Crusades, he never gave up the old gods (the bloody sort), and he likes to hear people scream. He leads a small, elite band of rangers who have similar motives. As a human, he'd be pretty nasty. As an elf, he's got the predatory instincts of a cat and has much more experience.
Not a Big Bad, but definitely a source of elves to fight.
Invaders
In another of my worlds, the elves hail from a distant island, but every so often a faction decides to leave and set up home elsewhere. Three thousand years ago, such a faction showed up elsewhere. It was actually a large group of factions; some of them had stopped on the plains behind them, and became known as the Copper Elves as their skins tanned under the sun. All but one of the rest of the factions stopped in a forest and named it Ithicaes. The last faction decided to keep going into settled lands; they were numerous and expert metalworkers, and possessed the secrets of vampire sorcery (which for them was simply a sorcery-induced battle frenzy, not a permanent affliction), and so were capable of surviving in new lands by force and by night. This brought them infamy; they became known as the Tasidhe or Dark Elves, and everyone dreaded their marauding approach.
The tasidhe aren't technically evil, but they're about as welcome as a longship with a dragon prow pulling up on the beach. If our Romans could fight the Germans for a thousand years or so, and neither side was convicted of being evil, you can fight the tasidhe. After all, they don't really care about your feelings...
Survivors
The world, for all intents and purposes, was destroyed. Clouds became so thick, and so omnipresent, that the sun became a myth. Horrible creatures lurked beneath the clouds, building their own civilisations based on slavery and foul magic. The elves couldn't fight that, so they left, heading for the highest mountaintops above the clouds. There on the snow and in their caves, they fought a bitter battle for existance; their skin was magecrafted to a deep black to fend off the sun's bitter rays, and they became warlike raiders, attacking the deep civilisations for necessities such as food and fuel. They could trust none but themselves, so they didn't. The black elves of the bright mountaintops will not be your friend.
These mountain elves can be adapted for any campaign with mountains; the core of their culture is an insular raiding mentality, where they have nothing so they must take it by force.
Variant: Light Mountain Elves
Sparkling with an inner light, these guys are native to high mountains, and use ensorcelled ice as their smithy material of choice. They're remarkably pretty, and when they come off their mountains looking for slaves and loot, they're operating in enriched atmosphere, becoming virtually unstoppable. I originally designed these as a replacement stock humanoid foe, albeit one slightly more powerful than orcs.
So there's some ideas. Hope some are useful.