What happened to Coyotito and who is responsible

This depends where in the novel you mean. Initially Coyotito is bit by a Scorpion. Further on in the novel Coyotito is killed by a bullet. We are unsure if it was a bullet from the tracker or a wayward bullet from Kino shooting at the trackers.

I think in a larger perspective one could hold Kino responsible over his obsession with selling the Pear for a high price. If it was not for Kino's resolve to undertake a dangerous trek with people chasing him, the tragedy might have been avoided.