Durkon needs character development. Turning a "very good" person with very strong principles, especially against vampires, into a vampire seems to be a pretty effective mean to drive it.
It's actually a classic of the vampire genre that opens a whole lot of things to happen.

Maybe Durkon dies now (for good and gets called back later), maybe he turns into a vampire, maybe he gets destroyed, maybe he finds redemption. In any case, I am pretty sure Durkon has opportunity to change.