2) Is this in direct response to what Robin says? I'm not sure in that case, so I'll guess. It seems to imply Robin has been impartial to Gisborne's death; that his death would not have harmed Robin -- it may even have been advantageous. He probably means to say it's an empty threat, since if it was up to Robin he already was a dead man walking.
In US prisons where the death penalty is still in effect, when the prisoner is being led from his holding cell to the place of execution, the phrase is announced to the other prisoners, "Dead man walking". There was a very effective movie a few years ago with this as the title. It was a general condemnation of capital punishment.