The Unforgiven by C. J. Cairncross
Cover shown is the 1978 first paperback edition by Sun Books (Aust). Cover art by Marilyn Newland.
"Was it the horror of Vietnam that drove young Michael Hackett mad? Or was it the corrupt brutality of his father, mighty industrialist Carl Hackett? Some demon was inside Michael screaming for revenge. So he teamed up with Arch Morley, a vicious, sadistic thug dishonourably discharged after a very nasty murder in a Saigon brothel. Together they pulled off a major heist: the payroll of Carl Hackett's biggest Sydney factory. But something went badly wrong. Suddenly it wasn't just robbery. It was multiple murder. And a siege. And a final showdown with death..."