He was played by Alan Rickman.
He hires Quigley to kill the Aborigines and tells him that his sharp-shooting skills are useful in doing the job. Quigley knowing Marston's true intentions turns down the offer and throws Marston out of his house twice. But Marston's goons beat up Quigley and Cora to the point of unconciousness and send them off to be dumped in the australian outback with no chance of living. Later Quigley and Cora save Aborigines from Marston's henchmen and Quigley kills 3 but the 4th escapes.
Quigley and Cora kill some more of Marston's men, who drove Aborigines off a cliff and save an Aborigine baby. A few of Marston's men recognize Quigley in a town and attack him. Quigley kills the henchmen except for 1, who he sends to Marston to let him know that he's coming for him. Marston after finding out prepares his men to kill Quigley.
When Quigley is captured by Marston, Marston and he face off in a quick draw gun shooting duel. Marston makes his first move but is shot and killed by Quigley who then kills Marston's remaining lackies.