Shakespeare most likely chose the word head instead of the word leader because it

emphasizes hamlet’s independence.
adds gruesome images to laertes speech.
continues his comparison of government to a human body.
creates a scary tone that contrasts with ophelia’s innocence.

read the passage from hamlet, act i, scene iii.

laertes: for he himself is subject to his birth;
he may not, as unvalu’d persons do,
carve for himself, for on his choice depends
the safety and the health of the whole state;
and therefore must his choice be circumscrib’d
unto the voice and yielding of that body
whereof he is the head.