Responses
The betrayer probably told the angels about the devil's plans and what they were doing before he decided to kill the protagonist. He just had to wait for the right time to kill him.
Prob Dantalion sided with the Angels and agreed to act as a spy and turn his back on Baal, he's prob been promised something to have caught his interest to turn his back of his kin.
Wasn't he winning the war for them? Why was he betrayed just as they were winning?