If we could only take one fact about tragedies to our desert island (don't ask), it would probably be—you can't escape fate. In Greek mythology, the concept of fate was personified by a trio of goddesses, and suggested that all people have a certain destiny that must be fulfilled. Essentially: no free will. Now, this whole fate/free will thing gets very (very) complicated for Christianity. Was Eve fated to eat that apple? If so, what does that say about God? Can we do anything to help our salvation, or are we just born saved or damned? For Samson Agonistes, the important thing is that Samson can't decide whether everything he does is fulfilling God's destiny for him (a kind of Christian version of fate) or whether he has total control over himself. So we're left wondering—where does his self-sacrifice fit in to all this?
Samson's belief that he might be fulfilling God's destiny isn't heroic or pious at all; it's totally narcissistic. Even in prison, he needs to believe he's special.
Samson has to be specially destined by God. Otherwise, how can you explain his whole hair-strength power?