These are the civilizations you don't want to face during, say, your next time travel adventure. And yes, the Spartans are down there.
#5.
The Celts
First of all, they had a thing for severed heads. After a long and trying battle they'd all unwind at the end of the day by collecting a few souvenir heads. Then they'd bring them home and decorate the house with them. So the average Celt home probably looked like a hunter's trophy room, crossed with the scene at the end of Halloween when all the mutilated bodies start popping up around every corner. Sweet dreams kids!
The reason for all of these head-chopping-good-times was that the Celts believed that the head held the soul, and so if you cut a dead guy's head off before all of that juicy soul leaked out of it, it was yours. A finders-keepers sort of deal, you might say.
A modern Celt.
Hey, remember Braveheart? And how batshit insane Wallace's army looked with their faces painted blue?
#4.
The Aztecs
The Aztecs believed that for every 52 years that passed, the world would end unless the gods were strong enough. And, as is common knowledge, the best way to toughen up a god is with a steady stream of constant human sacrifice (along with a dash of cannibalism, just for good measure).
So, Were They Really So Bad?
Yes. Yes they were. Did we mention the cannibalism? After they removed your heart and threw you down the stairs, they'd eat your arms and thighs, and whatever other bits looked tasty (the nuts? We do not know).
Some historians think that nourishment was the main reason behind all of this willy-nilly human sacrifice, as the Aztecs had no domesticated animals to slaughter, and as such their diet was low in fat and animal-stuff. You know how it is if you've ever had a roommate try a vegetarian diet. It's just a matter of time until you start waking up with teeth marks on your extremities.
And that's not even the bad part. The Aztecs held themselves to a high standard and for every situation they asked, "Could we make this more gut-wrenchingly gruesome?" So, for instance, during the sacrifice to the fire god, a newly-wed couple would be tossed into, you guessed it, a fire. Then, right before they finally died (from their horrendous burns), they'd drag them out, flesh still smoking, and dig out their hearts.
On one hand you might ask why none of the Aztecs thought this was odd. On the other hand, if somebody had an objection to the ceremonies we're guessing they kept that shit to themselves.
#3.
The Assyrians
Luckily for them (and unluckily for everyone else), just as that loveable sports team will sooner or later stumble upon secret weapon, a magic dog with an unusual aptitude for kicking field goals perhaps, after a couple of hundred years of being constantly harassed by more powerful nations, the Assyrians came across their own secret weapon. No, it wasn't Emilio Estevez, but it was nearly as powerful.
So, Were They Really So Bad?
Well, in a word, yes. Here's a cheery little quote from King Ashurnasirpal, who spent much of his spare time baking cherry pies for the homeless and teaching the neighborhood kids how to correctly adjust their bike helmets:
"I built a pillar over against his city gate, and I flayed all the chief men who had revolted, and I covered the pillar with their skins; some I walled up within the pillar, some I impaled upon the pillar on stakes, and others I bound to stakes round the about the pillar; many within the border of my own land I flayed, and I spread their skins upon the walls; and I cut off the limbs of the officers, of the royal officers who had rebelled."
"And then I was thinking about slap boxing with a lion, but only if there's time."
Sure, we understand that intimidation was how invading armies ended wars before they started, that this kind of psychological warfare was crucial for a conquering army. But reading that up there, we're pretty sure the Assyrians just enjoyed it.