I literally can't think of inherently evil reasons that don't boil down to the DM/author making it so.
You could try to use some biology to justify unpleasant behaviors. For instance, you could say that orcs are "evolved from" a fierce predator species (inasmuch as any D&D humanoid evolved from anything else) and are obligate carnivores. That would justify fierce or even violent behaviors... but even real-life solitary carnivorous animals like tigers aren't evil, and a social creature (like an orc would have to be) will still have to get along with others, at least others of its own kind, and they likely won't view every other intelligent creature as potential food.
Best I can think of is some sort of disease, like a rage zombie virus, that causes the infected to become ravening murderers but also causes the infected to not want to attack each other. Diseases and parasites are weird like that, so it's plausible. But if you decide that this disease only affects orcs, then you're back to DM fiat and trite motivations for evil. If you did this as an actual pandemic and any, or nearly any, race can become infected... then you have an AEKOS group without being having an AEKOS race.