The narrator is the murderer of a young woman so this: "She was so much more than they could ever understand, not a woman or a body or a piece of some puzzle." Is him saying how significant he considered the woman and how the police see her a victim/corpse (or a woman and a body as the murderer says) and, as this is what police do, they are concerned with figuring how her death happened (that's the puzzle part). This: "In death, she was a child. At the end, they all were." The murderer considers his adult victim a child - either because she was innocent, still young, or due to other features about herself. The second part is revealing two things. Firstly, he's killed more people. Secondly, that when someone is murdered, in their final moments they seem more like child (or at least, that's what I'm getting from the author using 'at the end' rather than 'in the end').

It means that to him she was neither a woman nor a piece of a puzzle, she was to him a lot as a victim, more than the police could understand. In other words, the police could not understand his real, deepest reason/motives for killing her. he thinks, basically, that the police do not understand him. In death she was a child = he sees her like a defenceless victim, as he sees all victims: defenceless. "In the end they all are" - is a common enough phrase which means something like "generally speaking, they are all..".

hello

Yes