In Tokyo Ghoul, fodder ghouls were getting killed by stabs to the chest and stomach, but then there are people like Nishiki getting impaled repeatedly and destroyed by Shuu, but he still lives. Plus, there's Kaneki getting those... injuries at the end of part 1, but he also still lives.
Is Ishida just being inconsistent, or is there a reason for this? What does it take to kill a Ghoul?
As you know, ghouls have very strong muscle and high regeneration abilities. A ghoul's body can be damaged by Quinque or Q bullets.
The main requirement to kill a ghoul are weapons like those mentioned above and critical damage to a ghoul's body that even their regeneration abilities is not enough to keep them alive.
As stated here regarding the weaknesses of a ghoul,
Ghouls will die if they receive extremely critical or life-threatening traumatic injuries that will kill them before they have a chance to heal.
As you described, some ghouls are still alive while some are not even when they received similar damage. It is because they are strong ghouls, they have more power compared to others and can handle bigger damage.
Whilst it's true that Nishiki should have died (as he hadn't eaten for quite a while), Kaneki was special since he was not just a one-eyed ghoul, which have unrivaled regenerative abilities but also had Rize's kagune, which has been described by Ishida as being a 'kakuja from birth', meaning it makes him stronger than ghouls in general. Whilst he did lose his memory, it is feasible that he survived multiple impalements by Arima.
