I think it's pretty clear that Cobb did kill his cellmate, but I read it that it was the only way Cobb could stop the cellmate from dragging Cobb down with him.
I don't disagree one jot.
But the distinction I was trying to make between this and what I saw in one of the flashbacks is that, like when my dog is panicking in the water and trying to climb on top of me for safety and thereby forcing me under and threatening my own safety (which has happened), what one of the flashbacks showed was that the cellmate wasn't clutching onto Cobb in a desperate attempt to save his life, but rather Cobb just simply holding his cellmates head down under water. There was no desperate, panicked grasping on the cellmates part in that brief scene.
Which may require handwavium on my part: There's what I understand to be the situation (being that Cobb had to choose between killing his friend or dying with him) and what was filmed (which was Cobb smothering his friend when it was clear that he wasn't being hindered or pulled down in any way).
I like the guilt and necessity that the understood situation implies, and the wonderful potential for future stories regarding that. And I think it really fuels Cobb's desire to change his life and do good. Not just for surviving the storm, but for surviving at the cost of his friend. Which is why I hope that the cellmate's GF and family come into the picture at some point. So that the change in Cobb's life isn't just a general thing, but also specific to what he did to get that second-chance at life.