Why was fate strong when it let Banquo's son escape from the murder scene in Macbeth?

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?