It's disgusting how Muslims get targeted (and Sikh by mistake) because a criminal is/was a Muslim. If a white non-Muslim commits a crime tomorrow I think I'm gonna start targeting all these white non-Muslim extremists. They're all the same just like all Muslims are the same.
And the Martin Place siege has nothing to do with Islam. It's just an idiot wanting attention and to speak to the PM waving the Islamic flag. If someone commits a crime tomorrow waving the Aussie flag I'll be sure to target all Aussies as they all are extremists for associating themselves with the flag of Australia.
It happens both ways because somehow it's easy to over-react then to use one's rational mind.
Like when white people attack or abuse Muslims, those who support the Muslim community point their finger and say white people are racist and ignorant, even though it's only a few stupid white people who do the abusing. Everyone likes to generalise, unfortunately.