Warning: this article discusses potentially disturbing violence.
Fans who've only seen the anime adaptation of Tokyo Ghoul will be absolutely floored to learn that Ken Kaneki essentially does the same horrific thing to Touka's brother Ayato in the manga as he did to his torturer Yamori in the anime. Although a horror story with a plethora of disturbing scenes, undoubtedly the most shocking of them occurs when the evil ghoul Yamori of the Aogiri Tree extremist group tortures the hybrid ghoul/human Kaneki for nearly 10 days. Even though Kaneki eventually gains the upper hand and unleashes his own horrors upon his former assailant, it's clear due to later events in the series that the hell he endured forever changed him. But what he does next in the manga diverges greatly from what happens in the anime.
Both written by mangaka Sui Ishida, the first season of the anime Tokyo Ghoul stays mostly true to his original manga. But by the second season, also known as Tokyo Ghoul √A, Sui takes the time to tell a completely different story. In √A, Kaneki actually joins the very group that mutilated him, killing alongside the Ayato who almost murdered his own sister and Kaneki's dear friend Touka. It's safe to say that Kaneki never tortures anyone ever again in the anime. The same can't be said about the manga.
By chapter 75 in the manga, Kaneki has survived the series' most disturbing moment when Yamori tortures him, in which he eventually turned the tables on his assailant. But then he hunts down Ayato before proceeding to beat him half to death. In a manner that would have made Yamori proud had Kaneki been his student in torturing, Kaneki first torments his victim by explaining all the ways in which someone can be beaten "half to death" before explaining what he eventually chose: breaking half of his bones. He proceeds to do just that, which readers see in full. There are also other instances besides this particular escapade where it's implied Kaneki tortures others for information or even fantasizes about the same.
All of these instances don't occur in √A and despite how disturbing they are, it's a shame that's the case, especially when it comes to Ayato's torture. For many fans, Kaneki's ultimate revenge of turning the tables on his own torturer was a defining moment in the entire series, and allowing Kaneki to indulge in them at least one more time - especially right afterwards - kept the horrific momentum going.
Additionally, it's clear by how events later transpire in both the manga and √A that Kaneki not only adopts Yamori's idiosyncrasies, mannerisms and twisted pleasures, but becomes the personification of his own torture when he takes the form of the very insect that Yamori used to terrorize him. It would therefore make perfect sense - albeit tragic sense - that Kaneki would transfer the horrors that he endured onto others. The Tokyo Ghoul manga takes this phenomenon to a completely new level by having Kaneki become Yamori in mere pages, exemplifying how profoundly the experience affected him for the worse.
Although Tokyo Ghoul √A still shows the mental decline of the tortured Kaneki, the full effect of what he experiences in the manga is lost. There are few things more horrific than torture, and seeing Ken Kaneki embrace such an inhuman pastime after undergoing the same horrors portrays him as a much darker figure than the Tokyo Ghoul anime ever could.