Well I mean, before that they weren't. They were simple but honorable and proud people who loved nature, the elements, their ancestors, and wanted prosperity. They were even pretty cool with the draenei landing on/living on their planet for a long time. It was Gul'dan, Kil'Jaeden and the demon blood all under the influence of Sargeras that twisted the orcs and led to the First War. (And before that, the war between the draenei and the orcs on Draenor.)

If we're gonna try and frame the orcs as simply just literally evil invading aliens, what about the night elves? A portion of them fucked shit up right good. Azshara and Xavius used the well of eternity to fuck everything up (even without the excuse of being literally tricked and manipulated into it. Azshara fancied herself as the consort of Sargeras, even.) while trying to bring Sargeras into the world.

That happened ten thousand years before the First War, and had dire consequences that have rippled down through the ages and led to many of the conflicts Azeroth has needed to deal with.