the slaughter of 20 million aborigines in Australia
There have never been 20 million aborigines in Australia. There are barely more than 20 million people in the country even now. The aboriginal population bottomed out at around 90,000 before climbing up to its current levels.
Aboriginal population prior to colonisation is estimated at around 250,000.
Like every other continent colonised, the vast majority of deaths were caused by introduced disease rather than by conflict.
There are a lot of cases where aboriginals were fought with, and where much wrong was done. In Tasmania, by 1840 every aboriginal had either been killed or exiled. If you want to excoriate the British for their treatment, fine, but at least learn some basics about the situation.