These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Gaunt's Ghosts
Complete Monster: Lijah Cuu establishes himself as a frightening, brutal soldier with a mean streak a few light-years wide to begin with. In The Guns of Tanith his true nature is solidified, when he rapes and kills a civilian woman and then kills Bragg for reporting him. He commits another unscrupulous act of violence in Sabbat Martyr, where he kills Colm Corbec while trying to kill Saint Sabbat. Though he is under the influence of Pater Sin's psykers at the time, that needs something to latch on to, like already-presenthomicidal desires. In Straight Silver, he almost beats Larkin (a relatively old man) to death for defending an innocent old lady and later tries to kill Larkin in revenge while the latter lies defenseless in the middle of a seizure, only to end up killing Sehra Muril, a fellow Ghost who just caught him red-handed. Cuu's actions naturally and effortlessly reinforce his nature as an absolute, unrelenting bastard.
There's also Guilder Worlin in Necropolis, who is a ruthless and petty businessman in Vervunhive. His very first appearance he waits at a trainstop (while the city is being bombarded) and when the doors of a packed train won't open to admit him, he and his bodyguards start slaughtering people until they all leave the train, allowing him to get on. He then leaves the fuel lines open after they've been ordered closed in order to get rich off the profits, and repeatedly both tries and successfully murders people in order to cover up his crimes. Thankfully he ends up being shot as everything is winding down.
Fan Nickname: Liijah Cuu is, quite accurately, referred to as Teamkill McBackstab von FethingBastard.
Magnificent Bastard It may turn out that Mabbon is a quadruple agent. First he leaves the Blood Pact to join the Sons of Sek so that he could get vital inside information on Sek's holdings. Then he defects to Imperium and gives them the information necessary to perform a raid on one of Sek's research facilities and steal tons of valuable data and research. However, it's implied that Mabbon has been loyal to the Blood Pact the entire time, and the entire operation was just a ploy to get that valuable data into the hands of the Blood Pact.
Moral Event Horizon Lijah "That Damn Bastard" Cuu crosses it when he rapes and murders a civilian woman in The Guns of Tanith.
Lets face it, he leaps gleefully over it in every book since his introduction, sometimes twice.
There's also Meryn's pension fraud racket, taking advantage of dead Ghosts and relatives for financial gain. Plus leaving Costin to die in order to keep him quiet.
Tear Jerker: Bragg and Caffran's deaths are probably the hardest hitting.
Values Dissonance: The morality and attitudes of the main characters owe more to 21st century "Western society" than that of the 41st Millenium Imperium of Man, likely in order to avoid making them all completely unsympathetic. Gaunt puts up with minor heresy, rarely executes soldiers, and isn't a religious fanatic - not the kind of behavior that is encouraged in an Imperial Commissar. More representative attitudes can be seen in Commissar Kowle from Necropolis, a Trigger Happy psycho.
That's only possible because not only Ghosts and numerous other regiments often display unbreakable morale, but the novels also downplay the threat of Chaos taint compared with a lot of other works in the setting - soldiers don't seem to be actually driven dangerously insane by seeing one Warp-horror too many, and it takes concentrated efforts on the parts of Chaos forces to corrupt large numbers of humans. So, there is no need for usual Imperial ways of keeping troops in fighting shape. That said, the Ruinous Powers are still quite potent, and the threat of Chaos corruption is brought up during and after the events of Traitor General.