The Fateless One lacked a destiny because it was already fulfilled, death being the fate of most mortals. The story being over, what was left was similar to the blank pages at the back of many books or an entirely new book.
The game's story actually is a pretty funny rip on the nature of video games, especially the Fae. Think about it: constantly repeating stories, doing the same thing over and over again...
The vast majority of the game's monsters are described as fae creatures. That's why they keep respawning—they're resurrecting. The mortal enemies are just being replaced by new recruits every few days. This also explains why they keep picking a fight with someone who has already killed hundreds of bandits before them.
The way destiny works in this game is basically the same history replaying itself over and over, due to the fae being reborn from their death and replaying the same role again, like actors replaying the same drama on a stage. Now imagine for a moment a well known play like Hamlet, except that one night, Yorik comes back to life and starts interacting with the cast despite not having any lines written for him at this point... who knows how the story will end as the other actors improvise around his unexpected interference?
When the Fateless One fights the ettin that was fated to kill Agarth the game gives you a free Fateshift kill, letting you Screw Destiny right in front of a Fateweaver who knows this isn't supposed to happen. This 'free kill' only happens at one previous point in the main storyline, during the tutorial when you're escaping the tower. But hey, it's a tutorial, right? Not in retrospect. That rock troll you killed at the end of the tutorial was clearly Fated to kill Fomorous Hugues, but you got to it first.