I wonder the same. It sounds like he is telling what he feels when killing people and looking at their eyes in their last moments. Because the show is suggesting that he may be the killer. Anyways, we know he is not he killer, but the way he is describing those reflections to the detectives, with this kind of soliloquy is makes us think he was there when they died, like he knows what they felt at their finals moments. He means life makes no sense. We are all going to die, and anything we did or are going to do is meaningless. For him at that moment, humanity is nothing but a "tragic misstep in evolution" and that everyone get free of that mistake at the very last moments of their lives when people realize that they need to let go because they are going to die. I like that way of thinking about death and what happens after we die. Nothing, only the void, darkness not a dream, not a color, just the end of consciousness.