Long ago the Aztecs founded their capital city (Tenochtitlan) in a place where they saw a hawk eating a snake on a cactus. They founded their city there because their god (Huitzilopochtli) told them that when they saw this they would found their city there.
The Aztecs believed that they should thank the gods for showing them where to build their home by taking enemy tribes warriors and sacrificing them to the gods and taking out their hearts fresh from their bodies, they also use their blood to sacrifice.
Before this happens, the warriors get to fight for their lives against people called jaguars and hawks that are great warriors armed with obsidian clubs.
The warrior would have to beat several of these people for his life. If the warrior wins which is really rare then he won't get killed and sacrificed.
Sometimes people who aren't from the enemy tribe won't be sacrifice and some other Aztecs would be chosen to be sacrificed.
For these Aztecs, this isn't brutal as it is their tradition and it is normal for them to do this.