By the time we reach the end of Kafka's The Trial, it's clear that Josef K has failed in his endeavors to prove his innocence. And that's not surprising, given that he doesn't know what crime he's supposed to have committed.
In a grotesque parody of how a normal criminal justice system works, it has been incumbent upon K to prove his innocence rather than upon the authorities to prove his guilt. Given that he hasn't been told what it is he's supposed to have done, K has naturally found this impossible.
The situation is further complicated by K's overriding feelings of guilt. He may not know what he's alleged to have done, but he still feels guilty all the same. And this overriding feeling of guilt undoubtedly plays no small part in K's meekness towards his executioners.
K is so overcome by guilt that he has effectively lost the ability to exercise free will. When the wardens pass the knife back and forth between themselves, they behave as if K has free will, expecting as they do that he will stab himself to death. But he's unable to do so, which means that the wardens have no choice but to kill him themselves.