Mafatu wants to kill the wild boar first of all because he is hungry, and second, because he wants to feel the pride and glory of having killed such a formidable beast all by himself.

As Mafatu is surveying the island on which he has landed, a wild boar startles him as it crashes through the undergrowth nearby.  A "smile cross(es) his face" as he imagines the thought of having the pig for supper.  He thinks about

"pig roasted underground in the hot oven stones...(and) his mouth water(s) at the golden prospect...He would make a spear and kill the (pig)!"

Mafatu, who had had a reputation of being not very brave among his people on the island of Hikueru, would never have thought "of killing a wild boar in single-handed combat" before.  To accomplish such a feat would be an amazing deed; Mafatu himself had never even known anyone who had killed a wild pig on his own, but he remembered that his grandfather, who had traveled as far as Tahiti, had told "how warriors of that island killed pigs in the mountains with naught but a knife for a weapon".  Mafatu thinks about what it would be like to kill a wild boar, and resolves to do it.  He says to himself,

"I will make...a necklace for myself from the tusks...and when I return to Hikueru men will look up to me and say: 'There goes Mafatu.  He killed the wild boar single-handed!'  And Tavana Nui, my father, will be filled with pride" (Chapter 3 - "The Island").