Goliath

We all know the tale of how David slew the Philistine giant, Goliath, with a well place sling-stone to the head, but is this how it really happened?

The KJV tells us, in 2 Samuel 21:19, "And there was again a battle in Gob with the Philistines, where Elhanan the son of Jaareoregim, a Bethlehemite, slew the brother of Goliath the Gittite, the staff of whose spear was like a weaver's beam."

The Torah as well as the NIV tell us however, "And there was again war with the Philistines at Gob; and Elhanan the son of Jaare-oregim the Beth-lehemite slew Goliath the Gittite, the staff of whose spear was like a weaver's beam."

This gets slightly more curious as 1 Samuel tells us that David took Goliath's head to Jerusalem. Why? The Israelites did not control Jerusalem at this time, so why take it there? Unless of course David was already King, had already taken Jerusalem, and his man, Elhanan, actually was the slayer of Goliath. Another possible reason would be that the name Goliath was actually inserted later, and was not actually the "Philistine of Gath", that David slew.

Why the reconstruction/ misinterpretation/ deception....and just who killed whom here?