Gunga Dass shot him with his own gun.

In my frenzy of delight at the possibilities of escape I recollect
shaking hands effusively with Gunga Dass, after we had decided
that we were to make an attempt to get away that very night. It was
weary work waiting throughout the afternoon.

About ten o'clock, as far as I could judge, when the Moon had just
risen above the lip of the crater, Gunga Dass made a move for his
burrow to bring out the gun-barrels whereby to measure our path.
All the other wretched inhabitants had retired to their lairs long
ago. The guardian boat drifted downstream some hours before,
and we were utterly alone by the crow-clump. Gunga Dass, while
carrying the gun-barrels, let slip the piece of paper which was to be
our guide. I stooped down hastily to recover it, and, as I did so, I
was aware that the diabolical Brahmin was aiming a violent blow
at the back of my head with the gun-barrels. It was too late to turn
round. I must have received the blow somewhere on the nape of
my neck. A hundred thousand fiery stars danced before my eyes,
and I fell forwards senseless at the edge of, the quicksand.

When I recovered consciousness, the Moon was going down, and I
was sensible of intolerable pain in the back of my head. Gunga
Dass had disappeared and my mouth was full of blood. I lay down
again and prayed that I might die without more ado. Then the
unreasoning fury which I had before mentioned, laid hold upon
me, and I staggered inland toward the walls of the crater. It
seemed that some one was calling to me in a whisper--"Sahib!
Sahib! Sahib!" exactly as my bearer used to call me in the morning
I fancied that I was delirious until a handful of sand fell at my feet.