4 ideas
A person can participate in violence not through a single decisive choice but through a chain of small accommodations, silences, and failures to leave — each step individually deniable. The ordinary man becomes a killer by degrees, never confronting himself with the full weight of what he is joining.
Mutual incomprehension turns into thefts, spearings and reprisals. Thornhill, who has resisted the hardliners, finally joins neighbouring settlers in a dawn massacre of a Darug camp. Afterward he prospers into a gentleman with a stone house, yet he cannot stop watching a lone surviving Aboriginal man on the land he took.
Settlers believed they owned land once they had fenced it, cleared it or planted it. Because of this they could not see the Darug's use of the same ground, their yam beds, fire-managed grasslands and seasonal camps, as prior possession. The settlers were not simply ignoring a claim they recognised. Their definition of ownership kept them from perceiving a rival claim at all, so Aboriginal presence looked like trespass on land already theirs.
People crushed by one hierarchy do not become allies of others at the bottom. They use a new frontier to climb, and the violence they endured teaches them how to use violence. Thornhill's London poverty and convict humiliation make land and status feel like survival, so he will destroy others to avoid falling back. Having been a victim does not protect him from becoming a perpetrator.