This is how I saw it.
Shane shot Otis to buy time to get away. Otis grabbed Shane almost immediately after being shot as Shane started wrestling for Otis' bag. Otis knew what was going on and tried to keep Shane there. Otis' gun goes off, and Shane grabs it and it and beats Otis with it. Otis has a fistful of Shane's hair and finally lets go, so Shane can get away.
It was pure survival, no revenge was involved at all. Shane even apologizes before he shoots Otis. If you look close, it even appears a few times that Otis is actually gaining ground on Shane, because Shane's gimp leg is slowing him down.
Watch again for yourselves:
Demise of Otis