Spoiler:
Meh, it's not like this is the first time Atlantis is destroyed. I mean, that is an excellent diversion.As for Namor teaming up with Doom...like the guy said, he is his only ally. X-Men? No help. Avengers? No. Fantastic Four? Nothing. Doom was with him from the beginning. Of course Doom has ulterior motives, and there is double-crossing, murder attempts at eachother and all manner of decieving and mind-games in their past...
But Doom has also saved Namor's life more then once (and vice versa), Doom saved each citizen of Atlantis from the endless coma they had, Doom saved Namor from his amnesia, Namor is pretty much the only person Doom has ever called friend...the fact is, Doom has historically been more an ally to Namor then anyone else in the surface.
I can't really blame him from allying with the guy. And this doesn't mean that he becomes a full blown villain either, he does have the best of an entire civilization in the mind.
As for killing his son, I don't see any bad thing happening there. The guy was responsible for deaths of thousands, tried to stage a coup in Atlantis and tried to kill Namor. Namor doesn't let blood ties hamper him when handing out cold justice (see Llyron who is related to him through his father), or when neccessary thing needs to be done (See Marrina). I think that was pretty badass scene, and it showed how ruthless Namor can be.
All in all, a good ending to a good miniseries.
I would have liked to see how Atlantean neighbours and protectorates respond to the destruction of Atlantis...etc. Lemuria and Mazikhandar.