Sure, it's a bit odd for that to end such a powerful being, but it was still very epic. A battle between father and son? Nothing more epic than that!
But how many of the other Good/Neutral NPC's meet such unhappy ends?
Of the neutral NPCs, the only one that gets a happy ending, relatively speaking, is Jaheira, and then only if she the pc declines godhood. Jan gets busted smuggling monkeys, don't remember what happened to Haer'Dalis, but it was probably predictable, Viconia doesn't fare well if she's neutral, we all know what happened to Yoshimo, CN Anomen doesn't fare too well, don't remember who else was neutral.
All the evil NPCs get bad endings.
I see personally see the ending as a very poor attempt at tying up loose ends. Unfortunately it is only believable if one is willing to believe that Cernd was such a terrible father - I don't, despite Bioware having him conveniently forget all about the child after he is left at the Druid Grove.
I don't know. Ash was never Cernd's top priority from his conception to his death. In the entirety of BG2, the only time he ever mentions his son is during the quest. Once the quest is over, his son is forgotten. Maybe he *did* intend to go back to the druid grove and raise his son properly. Ash was only 2 when Cernd reclaimed him. It typically takes me 3-4 months game time to complete the entire game, so he would have been able to develop some kind of relationship with him. But patterns are patterns. If Cernd was willing to go the 3-4 months without giving his son another thought, maybe he went the next 18 years the same way, remembering in fits and spurts, but always being called away to serve Nature.
I'd like to see Cernd maybe mention once or twice that he wonders how his son is doing. Maybe something in the underdark: "I am glad Ash will not have to face the endless dark of these cold stone halls." Odd bits and pieces to let the player know that Cernd has not forgotten his son.