Why does Golding choose to end Jack and Ralph's relationship with Jack trying to kill Ralph in Lord of the Flies.

One of the themes of Lord of the Flies
is good v. evil and Jack's attempts to kill Ralph exemplify that theme. 
Ralph is good and represents the idealist while Jack represents what Golding felt was
the true, inner human being - the evil inside all mankind.  Golding believed that
without society's constraints to make people conform to certain rules of behavior,
people would become savage and brutal.  He places the boys on an island, free to create
their own society and their own rules.  He shows them becoming increasingly savage to
the point where they are killing each other.  First, Simon is killed while the boys are
in a heated frenzy, then Piggy is deliberately killed by Roger.  Finally, Jack and his
tribe try to kill Ralph but the British naval officer shows up on the island to rescue
the boys before the boys can finish off Ralph. Throughout the book, Ralph tries to lead
by pleading with the boys and by trying to appeal to their sense of right and wrong. 
Jack, however, leads by first, enticing the boys with promises of food, fun, and
protection, then he leads by intimidation and threats.  The two boys are opposite ends
of the spectrum and Golding believed that the evil end is the one that would win, thus
Jack's attempt to kill Ralph and completely erase any good left on the
island.