Monster Island - Christopher Golden

When Angel first came to Los Angeles, it was the fast-talking, half-demon Irish-American Doyle who first explained to him the mission for which he was destined: the saving of lost souls. All the more ironic, then, that Doyle himself was the one Angel couldn't save. Reconciled at last to the heritage he had long denied, Doyle sacrificed his life to save a shipload of helpless, peaceable demons from a Nazi-style extermination crew.
But Doyle's legacy lies not only in the gift of visions he bequeathed to Cordelia with their one and only kiss. Unaware of their danger, Angel and co. are unprepared for the visitor at their door. Intending to free him from what he sees as the human taint that has always held him back, Doyle's demon father, Axtius, is none too pleased to find instead that his wayward son is dead.
Meantime in Sunnydale someone apart from the Slayer is slaying. The victims, strangely, all appear to be human-demon half-breeds. The trail leads Buffy to Los Angeles, where before long she and Angel find themselves in the front line to stop an avenging fury from wholesale slaughter...