The Old Firm

Canon states that they burned Troy and brought the Black Plague to Flanders. Other possibilities include:

Killing the last mammoth and Neanderthal
Invented assassination
Found Socrates hemlock, 399 BC
Burning the Library at Alexandria, Circa 48 BC
Provided the asp for Cleopatra, 30 BC
Crucifying Jesus (Hinted at in canon), between 26 to 36 AD
Massacre of the Latins, 1182
Aztec sacrifices, Circa AD 1487
Roanake Colony's disappearance, AD 1587
Joined Ottoman revolt in Egypt,  May 1773 AD
The French Revolution, AD 1789–1799
Joined Donner Party, AD 1846–1847
Bored holes in several of the Titanic's lifeboats, April 1912 AD
Fed on corpses at Auchwitz, 1939-1945
Gave Charles Manson a copy of the White Album, November 1968 AD

"You know, you really should've gotten the hint when we bombed the mailing center," sighs Mr. Croup, absently wiping blood from his ax. "But you just had to keep on sending this stuff."

The head of the Direct Marketing Association whimpers. "It's just junk mail, you didn't have to get so upset!"

"We like getting upset. 'S a hobby of ours," explains Mr. Vandemar, helpfully.

The morningstar comes down.