Basically, that idiot priest seeking death to himself since he didnt aware what he face is the strongest one ever in history due he can eliminate LVL 5 threat all by himself.
I dont know what he gonna do with Selka, but assuming from his act, he want to save her before her entire body irreversible since it takes time to degenerate living object to monster and she just transmute recently which is there is a chance to save her
Remember, this is NOT A SPOILER, just a random opinion