Was it the power of fate that allowed Fleance to escape?
If you believe that the witches knew and controlled the future, then maybe you believe it was fate that made it possible for Fleance to run away while his father was being murdered.
Or maybe it was just dark, and Fleance was small and hard to see, and he was able to quickly heed his father's dying words, "O, treachery! Fly, good Fleance, fly, fly, fly!"
Certainly, things seem to work out the way the witches had predicted, and maybe they knew the future... these facts would seem to give strength to the concept of fate.
Or perhaps it was all just luck and chance.... who can know for sure?