Psalm 23 in a different light

Many years ago, I participated in a study of Psalm 23.  Once every week for the three month study time, we recited this Psalm together.  It stuck in my memory!

What also stuck was a few oddities about the song (psalm).  It's beginning is all about God. Then it's focus suddenly seems to shift to another being, at least in reference.  Now it seems as if God is being compared, in a sense, with another being or beings the writer is afraid of, or could be afraid of.  

I always wondered about verse 4 and 5.  What was the significance of 'the valley of the shadow of death' and why was the psalmist insistent that he would not be afraid of it? What did the rod and staff have to do with anything?  Was God going to beat up enemies in the valley?  Could God not protect the Psalmist in the dark valley?  The Psalmist's language seems confident, maybe even overly so, but it almost sounded like bravado.   As if he had to reassure himself by his words.

So after many years, and in a more Torah-knowledge based understanding of 'satan' and 'ha-satan' that is fairly recent, I took another look at this Psalm.