Quigley Down Under

Matthew Quigley (Tom Selleck) is an American cowboy and sharpshooter with a specially modified rifle with which he can shoot accurately at extraordinary distances. He answers a newspaper advertisement that asks for a man with a special talent in long-distance shooting, using just four words, "M. Quigley 900 yards," written on a copy of the advertisement that is punctured by several closely spaced bullet holes.
When he arrives in Australia, he gets into a fight with employees of the man who hired him, who are trying to force "Crazy Cora" (Laura San Giacomo) onto their wagon. After he identifies himself, he is taken to the station of Elliot Marston (Alan Rickman), who informs Quigley his sharpshooting skills will be used to eradicate the increasingly elusive Aborigines. Quigley turns down the offer and throws Marston out of his house.
The aborigine manservant knocks Quigley over the head and Marston's men beat him and Cora unconscious and dump them in the outback with no water and little chance of survival. However, they are rescued by Aborigines. Cora now reveals that she comes from Texas. When her home was attacked by Comanches, she hid in the cellar and accidentally suffocated her child while trying to prevent him from crying. Her husband had then put her alone on a ship to Australia. Now Cora consistently calls Quigley by her husbandâs name (Roy), much to his annoyance.
When Marston's men attack the Aborigines who helped them, Quigley kills three. Escaping on a single horse, they encounter more of the men driving Aborigines over a cliff. Quigley drives them off with his deadly shooting and Cora rescues an orphaned baby she finds among the dead Aborigines. Leaving Cora and the infant in the desert with food and water, Quigley rides alone to a nearby town. There he obtains new ammunition from a local German gunsmith, who hates Marston for his murdering ways. Quigley also learns that he has become a legendary hero among the Aborigines.
Marston's men are also in town and recognize Quigley's horse. When they attack, cornering him in a burning building, he escapes through a skylight and kills all but one of them. The injured survivor is sent back to say he will be following. First Quigley returns to Cora and the baby, which she has just saved from an attack by dingoes. At first she had tried to stop it crying, but then told it to make as much noise as it liked as she gunned the animals down. Back in town, she gives the baby to Aborigines who have arrived to meet up with them after Quigley tells her that the child has 'a right to happiness'.
The next morning, Quigley rides away to confront Marston at his station. At first he shoots the defenders from his location in the hills but is eventually shot in the leg and captured by Marston's last two men. Marston, who has noticed that Quigley only ever carries a rifle, decides to give him a lesson in the "quick-draw" style of gunfighting. As the two face off, Marston makes the first move, but is beaten to the draw by Quigley, who shoots the two remaining men as well. As Marston lies dying, Quigley refers to an earlier conversation, telling him, "I said I never had much use for a revolver; I never said I didn't know how to use it."
Marston's servant comes out of the house and gives Quigley his rifle back, then walks away from the ranch, stripping off his western-style clothing as he goes as the other two Aborigine servants also leave. Some time later, an army troop arrives to arrest Quigley for murder until they notice the surrounding hills are lined with Aborigines, having been gathered by the former manservant, and decide to withdraw. Later he and Cora book a passage back to America in the name of Roy Cobb, Coraâs husband, since Quigley is still wanted. On the wharf she reminds him that he once told her that she had to say two words before he would make love to her. Smiling broadly, she calls him "Matthew Quigley" and the two embrace for the first time.

Question: What did the army troop want to arrest Quigley for?
Answer: murder