You don't need to be alien to be scary. powerful pillaging hordes are perfectly fine.
Not all threats need to be existential, physical is perfectly fine as well.
Initially, they were Chaotic and not Evil: "They have bad tempers and do not like other living things; they will often kill something for their own amusement... The orcs are satisfied by being allowed to kill and burn as much as they want."
They became Lawful Evil in AD&D: "Orcs are cruel and hate living things in general, but they particularly hate elves and will always attack them in preference to other creatures. They take slaves for work, food, and entertainment (torture, etc.) but not elves whom they kill immediately."
But let's face it, they were evil at start because they were like LotR, with Sauron replaced by evil orc gods. And this has been perpetuated even down to 5e: "The evil deities who created other races, though, made those races to serve them. Those races have strong inborn tendencies that match the nature of their gods. Most orcs share the violent, savage nature of the orc gods, and are thus inclined toward evil. Even if an orc chooses a good alignment, it struggles against its innate tendencies for its entire life. (Even half-orcs feel the lingering pull of the orc god’s influence.)"