It's not so much letting him go as not letting him be forgotten. If someone else was actually responsible for his death, then Mark deserves justice. Just because he was alone and not significant to society doesn't mean his death is less worthy of being investigated. The arson investigator and police detective shouldn't be allowed to sweep his death under a rug just because no one is pushing for an investigation.
If nothing criminal happened, then fine, just let us know. Otherwise, it seems that no one could be bothered to follow up on the death of a poor, deaf guy with issues, alone in a big city. He didn't even get the dignity of having a name in the newspaper. He was just a "body" that was found.