This is a SIMILE - if words such as 'as' or 'like' are used in order to compare two things (in this case, 'giant rocks with razor edges crouch LIKE a sea monster with wide open jaws'), then it is a simile.
In deciding destiny, I think both are important and play a significant role. Each person's destiny is decided by what he is and what he does; and what every person is to be or do is determined by what he lives for, thinks for, or works for.