one of the prophecies says that the lightbringer will kill his brother. We have learned that Andross is kip's actual father. Zymun is not his brother, Gavin is. If Kip is the lightbringer than he will have to kill Gavin or maybe Sevastien will return to be killed by his hand.
I think Gavin killed his brother Sevastien under the influence of black luxin the first time he drafted it.
that was the starting of his obsession with hating the wights. The dead man in the black prison cell may have guided him from early on and may still be lying to him about it. Maybe he will learn about it when he goes to kill orholam. I will hate for him to die at kip's hand...
Why Gavin(Dazin) killed his younger brother?
I think it had to do with the prophecy "Of red cunning, the youngest son, Will cleave father and father and father and son." Perhaps he wanted to kill the lightbringer under black's influence.