Lord Tywin knows the value of having unscrupulous men in his employ—namely, that if he is paying them, they tend do bad things to the people who are not signing their paychecks. Some of these lowly fellows are:
Ser Amory Lorch is one of Tywin Lannister's bannermen. While on a raiding party, he encounters Yoren and his recruits held up in a hold, and although Yoren claims he has no part in the war, Amory attacks, leading to the death of several men, including Yoren. Clearly, Amory is not a stellar guy. Later, Amory is made castellan of Harrenhal and is captured when Roose Bolton takes the castle. Bolton has him executed by being put in a pit with a wild bear—Arya thinks there is a sense of poetic justice to this since the bear looks like Yoren (48.Arya.192).
Polliver is one of the Mountain's men. He takes Needle from Arya (27.Arya.22), and for the offense, she adds him to her prayer-kill list. Although she's got his number, Arya doesn't get a chance to kill him as he marches with Lord Tywin's host.
Raff is one of the Mountain's men. He puts a spear through Lommy's stomach and kills him, so he won't have to carry the boy back to camp (27.Arya.24). For this, you guessed it, he lands himself on Arya's prayer-kill list. Raff also marches with Lord Tywin's host.
Yet another of the Mountain's men, but this is the one who doesn't get away. While staying at Harrenhal, Chiswyck tells the story of how the Mountain raped a girl because her father annoyed him; he laughs as though it's the funniest thing in the world, and Arya overhears (31.Arya.61). She whispers his name in Jaqen H'ghar's ear, and Chiswyck has an unfortunate "accident" off the wallwalk. The night he dies, Arya thinks how "there was one less name to hate" (31.Arya.68).
Weese is an understeward at Harrenhal and Arya's taskmaster. He hits "at the slightest provocation" (31.Arya.8) and will set his dog on anybody who annoys him. After pushing Arya too far, his name just happens to be whispered in a certain Jaqen H'ghar's ear—and Weese's dog "mysteriously" rips his throat out later. Go figure.
Lord Vargo Hoat is the leader of the Brave Companions, a group of sellswords hired by Lord Tywin. The group has earned the nickname the Bloody Mummers. Vargo brings a group of Northern "captives" to Harrenhal with the secret intent of seizing the castle for Roose Bolton. With the help of an unknown Arya, the Northmen take Harrenhal as planned (48.Arya.136). Vargo Hoat then joins Roose Bolton. He has a speech impediment.