Residents of the world of Tokyo Ghoul have to deal with the co-mingling of both ghoul and human lives. Most humans despise ghouls and their power, which is why the CCG is formed. But while the CCG investigates and hunts down ghouls, these beings lives' are still shrouded in secrecy. Some of them blend into human society and avoid trouble, while others prefer to lurk in the shadows and lash out at humans because a crucial part of their existence means feasting on flesh and blood. Some find they have no other choice but to succumb to this need, and others even use it to become stronger by cannibalizing their own.
Though it seems willfully evil, when you think about it, ghouls actually don't have much of a choice when it comes to their consumption of others -- even other ghouls. Societal pressures from both sides force them to become stronger in order to survive, which is the ultimate knife twist.
Of course, there's still the argument that eating normal human food doesn't outright kill a ghoul. There are several times in the series in which Touka eats the homecooked meals of her friend as a part of her commitment to blending into society. However, ghouls become nauseous after eating human food. They can disguise their need to throw up, but in the end, they still require flesh and blood for sustenance.
While an ordinary ghoul can quietly live off humans, this pathway doesn't guarantee more power. In a world where the CCG actively hunts ghouls and many ghouls with more power push around those beneath them, it benefits a ghoul greatly to empower themselves. To fulfill this need, ghouls have the option to feed off of each other, an act that can be dangerous for all parties involved. If a ghoul cannibalizes enough times, they can end up transforming into a deranged kakuja. While a kakuja is stronger than the ordinary ghoul, they also usually lose their mental capacities. The worst part about becoming a kukuja through ghoul cannibalization, though, is that it is actually a rare feat, meaning that it's a big risk with only a slim chance of a reward.
This journey is one that Ken Kaneki takes over the course of Tokyo Ghoul's later seasons. After feeding off of his first ghoul targets, he gains the power of an incomplete kakuja. Wearing a mask that covers half of his face, along with his tail-like appendages turning into centipedes, he gains a lot more mobility than he had previously.
The cost of this power is having little control over himself. In his berserk state, Kaneki ends up fighting against members of the CCG in a ghoul-detainment facility alongside members of Aogiri. In fact, Kaneki only escapes his first fight as an incomplete kakuja due to the interference of an Owl, a third party that takes him out of the facility entirely.
As if gaining the powers of a kakuja and having some control over them isn't a hard enough task, the process is full of risks that would be difficult to get past. The CCG is a threat that any normal ghoul has to deal with on a regular basis, but this threat becomes even more relentless the more that a ghoul tries to amass power. For instance, the CCG develops files for many high-priority ghouls such as Kaneki, who is referred to as 'Eyepatch' by the investigators. Assuming that a ghoul could avoid the CCG in its rise to power, there is also one other problem they have to contend with.
Not only would an empowered ghoul have to deal with threats from human society, but some ghouls don't take too kindly to others rising up the ranks. Depending on where a ghoul decides to hunt for lesser ghouls, they could end up in trouble with even more powerful foes, such as Anteiku, an organization that decreases the number of ghoul attacks on humans in its district. (Although, if a ghoul did acquire kakuja powers and manages to control them, they could better protect themselves against such dangers.)
Ultimately, ghouls face a tragic fate in this universe as a result of the blended society they live in. While trying to join a ghoul organization may grant a ghoul some security, this is, at best, a temporary measure. Depending on the group, they may not have the interest of individual ghouls at heart. Therefore, the best way to ensure one's safety is to grow stronger personally, a choice that necessitates eating one's own, leaving the ghoul world forever divided in an already-divided world.