I agree 100% with everyone that finds what happened to Stingray awful and something that Namor would not do.
Besides the general reprehensibility, there's the fact that Diane Arliss Newell was something of an old flame of Namor, and Namor has a long history of bending over backwards to keep his old girl friends happy. Murdering Diane's husband Stingray, the father of her children, will not make Diane happy.
I am with those that think Namor is not himself, in part because of these reasons:
1) Namor has never had completely black eyes -- except for Shark Namor, from Sub-Mariner the Depths, which isn't 616 Namor.
2) Namor would never call Tiger Shark, who participated in the murder of Namor's father, brother.
3) It is clear that Namor hated being possessed by the Phoenix, so why would he wear an outfit that is basically a variation on that?
4) While I liked finally getting Namor's actual thoughts as narration, they weren't exactly consistent what Namor has thought and said before. Namor more often thinks of himself as a bridge between the surface world and underwater world, rather than the ruler of both. Neptune himself told him that. So his "holy war" isn't exactly endorsed by Neptune.
5) I think Namor doesn't get enough credit for his strength and abilities by modern writers, especially underwater. He should be unbeatable underwater. That said, he's not the Hulk. Anger, righteous or otherwise, doesn't make him stronger. There's definitely something off in his fight with Avengers.
6) Hungry Atlantean children are NOT going to runaway from Namor, or a meal. They were obviously scared of something we aren't seeing, that drove them to basically kill themselves on a surface beach, rather than face underwater.
7) Except for Andromeda, everyone on Defenders of the Deep is a Namor foe -- though I'm not entire certain of the identity of two of them (the white haired guy above Man O'War and the crustacean), they don't exactly look like friends. I'm wondering if they are Suma Ket and one of these guys: