Question: Why did the Aboriginal manservant hit Matthew Quigley on the head when he threw Marston out of his own house, after telling Quigley he wanted him to kill aborigines?
Answer: In fact, it was more likely that in the beginning of the film he was in more of an "Uncle Tom" (for lack of a better term) and hits him because he feels he should help his "master." He later feels bad and by the end he has come to his senses which is why he doesn't make the same mistake twice.
More like he was emboldened because his master is dead. If not Quigley would have got a second thrashing from the aborigines.
Answer: He felt that if Quigley fought Marston he might have been killed. He hit him to save his life.
SantaJim