You sit are correct. Even if you just read it, without analyzing the word definition (see above), in context it only makes sense that he felled Goliath with the stone, and killed him with the sword. Digging deeper will still provide more evidence of that. The answer is simple, he knocked him out (prevailing in the contest), and cut off his head (killing and taking his trophy).
sit=Sir. Sorry for the typo.
I don't see it this way. The text says; verse 50, "he struck the Philistine and killed him; but there was no sword in David's hand."
Read on, verse 51. The cutting off of the head was to convince the army of the Philistines that their champion was dead.
The stone killed him, the decapitation proved it to his army.