Smasher Sullivan is my least favorite character in The Secret River, and I'd guess that I'm probably not alone on that. With a name that sounds like a villain in an old western, he appears to be the archetypal character for the reader to root against.
I think Smasher Sullivan carries many different significances in The Secret River. For one, his explicit racism, sexism, pride in violence, and barbaric acts contrast him from the more divided Thornhills. I wonder if Grenville created Smasher Sullivan's character almost as a way of reassuring herself that while her own ancestor did horrible things to Aboriginal people, he could have been much worse. The Thornhills see themselves in the same way.
This same logic persists today when people try to convince themselves that they are not really racist, sexist, homophobic, etc. because they aren't 'as bad' as someone else. Besides being outright false, this perspective is extremely dangerous because it masks accountability and recognition of one's own biases. Perhaps this message is part of what Grenville is trying to communicate as William Thornhill is initially disgusted by Smasher Sullivan's treatment of the Aboriginal people but slowly is more and more tempted to commit violence against them himself.
The other quality that I believe Smasher Sullivan represents is the duality between white people's hatred and fetishization of native people. While seeing the Aboriginal people as sub-human pests, Smasher Sullivan also finds an Aboriginal woman attractive enough to make her his sex slave. That entire scene made me nauseous, but it also made me think about the way white people have appropriated native culture and regalia throughout history. It is as if white people want to extinguish native culture and can't get enough of it all at once.
So while Smasher Sullivan seems like the obvious villain of The Secret River, I think his character is much more nuanced and representative upon closer inspection. I hope the Thornhills are able to stay away from him as the book continues.