Homer's Achilles, we suppose he added to the existing ancient legend, is a turning point in Western civilization in that Achilles is given a destiny but he also has a choice through his own action or inaction between two outcomes, so even in a culture of intervening-meddling gods, Man has effective freewill.
Free will is one of my options. One of my two MCs repeatedly has the ability to walk away, but believes so strongly in his course of action, that he ignores the ever-increasing insanity of his quest. Another option is to say that the MC was destined to lead the quest, though not necessarily succeed in it.
The other one I'm exploring is the case where the MC was never given a choice, but is trying his best to avoid his fate, which he believes would lead to all-out war for the human race. So far, he keeps failing to divert away from that destiny.
Finally, there is a very real possibility that either or both destinies are just imagined.
Eventually, it all collides. Should be a blast. Pun intended.