In chapter four of Call it Courage, Mafatu begins to make various tools to help with his survival. He finds a whale bone and carves it into a knife. He is happy he has spent so much time in his past crafting items, as it is certainly coming in handy now.

While out checking fish traps on his raft, he notices a tiger shark that is stealing his fish. It angers Mafatu, and it is at this time that his dog, Uri, barks and falls into the water. Mafatu rushes in after Uri, diving into the water and stabbing the shark with his new knife. The blood from the shark draws in other sharks, and they tear it to pieces.

Chapter four shows Uri becoming more and more confident in what he has been able to accomplish so far on his own. Killing the shark to save Uri is just one of those things.

In Chapter 4 of the book, Mafatu at least causes the death of the shark.  He does not necessarily kill it all by himself, but he does have a big part in its death.

A few pages before the shark dies, Mafatu has found a whale skeleton and has used some of the bone to make himself a knife.  When Uri falls off the raft and the shark comes after him, Mafatu dives in holding his knife.  He stabs the shark in the belly.  Blood rushes from the shark and other sharks tear it to pieces. 

So Mafatu kills the shark by stabbing it in the belly, though the other sharks play a part in killing it.