There's a difference between someone who has a lot of determination and someone who just doesn't know when to quit. The Black Knight really should have stopped while he was ahead (by which we mean he had all or most of his limbs intact).
When we first see him, he appears to be a very powerful and mysterious man. He remains silent even in the face of Arthur's generous offer. In fact, he even refuses to move for the king (which also brings us to the question of why he's guarding such a small, insignificant bridge).
He continues with a still stoic "'tis but a scratch" when he loses his first arm. Even by the time he's completely limbless he's screaming
BLACK KNIGHT: Oh oh, I see, runnin' away hey? You yellow bastards! Come back here and take what's coming to ya!
Like Tim the Enchanter, the Black Knight episode is a parody of a stereotype—this time, of macho invincibility. The Pythons make quick work of destroying that stereotype.