Okay, I interpreted the ending in a very different way than everybody else.
To me, the lines
"Tyler made the sign of the old ones again, dabbing the second finger of his left hand to his forehead and making the circle. The priests had killed him too, but Una, at least, would be safe"
meant that Tyler had been taken over by Una, so Tyler's mind (but not his body) was effectively killed, and Una (whose mind now inhabited Tyler's body) would survive. For me, the clincher was that Tyler's body made the religious sign as if he was a frog-person.
But maybe I'm wrong....