There have been many civilizations in history, but there are a few that are the most destructive, most abusive, and most horrifying enemies that anyone's ever had to face. Which of them is the most vicious?
In chronological order:
1. The Islamics
While I'm just about sure this is going to trigger a flame war, the Islamic Empire deserves to be on this list for one thing and one thing only: speed. After Mohammed's journey to Mecca in 632, the Muslim's carved an Empire stretching from Spain to India, dwarfing the Roman and Alexandrian Empires that preceded them. No one can doubt the devotion these warriors had to their faith. While Christian and Jews were respected as "people of the book" (though they were basically thrown into the ghettos of new towns in the Islamic Empire) any pagan who refused to bow to Mecca was brutally slaughtered. They reached their height upon a raid into Frankish territory in 732 at Poitiers.
2. The Mongols
A gathering of tribes in what is today a backwater third world country, Temijin, a.k.a. GHENGIS KHAN, transformed these yak herders into an army of modern-day proportions. The sight of their cavalry instilled fear in the hearts of every king from China to Austria. Their sheer brutality can be represented by their complete destruction of the Caliphate: upon their arrival in Baghdad, they "respectfully" trampled the Caliph in a burlap sack to prevent the shedding of his blood. The Mongol Empire toppled virtually every nation they challenged (the only exceptions I can think of are Japan, Vietnam, and Austria). Their Empire reached its peak when the Horde was spotted outside the walls of Vienna in 1242. The attack was expected to come any day, and defeat was certain, but Ogadai Khan died and the Horde ended their conquest.
3. The Mexica (arguably the Aztecs in general)
A short lived Empire, the Mexica ruled from their bridged city of Tenochtitlan. At its height in 1520, it rivaled most cities in Europe in sizeand power. The Aztec people however, were blood thirsty. It stemmed from their religion, which dictated that if human sacrifices were not paid to the gods every morning, the sun would not rise. Aztec mythology represented the sheer destrutive mindset of the Mexica pople: the earth had already been destroyed multiple times, and was soon destined to fall to deadly earthquakes. Their destruction was brought on by Spanish conquistador Hernan Cortez. When he arrived in Mexico, the Aztecs believed him to be a god for a multple number of reasons. They welcomed him to Tenochtitlan only to later regret it as the Spanish technology, dieases, and their ability to pit the Mexica's enemies against them ultimately destroyed the Mexica nation.
4. The Third Reich
Arguably a "great" civilization. But their 12 year long persecution and Final Solution of the Jews puts them on this list all by itself. What puts them here would also be their Russian offensive. Hitler's sheer hatred for Communists and Jews brought about his downfall. The invasion of the Soviet Union would ultimately prove to be fatal for the Reich, and shuttling and murdering countless Jews would be a complete waste of resources only bringing his destruction faster.
5. The Japanese Empire
An ally of the Third Reich, the Japanese built their civilization on a code of honor and a racial attitude similar to Hitler's Third Reich. The Japanese believed they were highly superior to the Chinese, and began their invasion via Manchuria in the 1930s on said premise. To simply sum up the viciousness of the Imperial civilizatin without discussing their suicidal battles in the Pacific: the Rape of Nanking.
6. Other
There are other civilizations which could easily be on the list of top 5... certainly top 10. Off the top of my head: the Vikings, the Russian Empire, the Spanish Empire, the Ottoman Empire, and even Wallachia.