I'm not sure whether groups and classes of people can be blamed for the actions of lone gunmen. But if we are going to go down that road, we should be consistent. Some would like to blame all Muslims for the actions of a few, while not blaming all White Americans for the actions of a few. Meanwhile, others reject the notion of collective Muslim guilt, even while they insist on the notion of collective White guilt.
The same is true for rhetoric. If we are going to argue that Trump's rhetoric causes right wing violence, then we should ask who's rhetoric causes left wing violence. It Trump's demonization of Muslims causes anti-Muslim violence, then it follows that demonization of police causes anti-police violence. On the other hand, if anti-police rhetoric doesn't cause anti-police violence, then neither does Trump's anti-Muslim rhetoric cause anti-Trump violence.