Why were ancient Peruvians so bloodthirsty?

They did do it a fair amount. The Moche are actually one of the most epic examples of this sort of thing that you can find in the world ever. Some cultures did this to a certain extent and had sacrifices in a more modest way.

The Incas used to bury children on mountain tops for example. But the Moche were really really sanguineous. They had a little tiny area on the coast of Peru and they hacked it out of virtually nothing so it was very small with lots of canals and agriculture.

The society was very rigid and they would go out to fight other tribes and nick all their land and so on.

They would then ritually decapitate all of the captive men. They sometimes kept some of the women, but not very often.

They would cut off their arms and remove bones from their arms while they were still alive. They would smash bits of them off and then throw then off these cliffs into an enormous pit and leave them there for the vultures to eat.

They were pretty nasty! They even had a deity called the decapitator god, which had a giant eagle-like head and a nasty blade in his hand. He would go around decapitating all the captors.