Donald Trump views the lack of police force to halt protests that have turned violent in American cities as a willful surrender on the part of governors and mayors. He views this lack of action as weakness and has very publically voiced his opposition to this inaction on the part of local government, encouraging them directly to react forcefully against looting and destruction of property. His encouragement has included calls for tracking, arresting, and imprisoning protestors for up to 10 years.Trump believes that anyone, police or civilian, who takes a strong, demonstrated stance against those involved in current protests are doing their part to help bring law and order back to the country. His stance on quelling protests comes from a desire for peace across the country, and he has the support of many American citizens on this topic. 
Donald Trump assumes all protestors are causing violence, despite evidence to the contrary. Not only is this rhetoric hostile towards law-abiding citizens exercising their constitutional right to peaceful assembly, but it is also highly destructive towards country unity as it appears to encourage police brutality against peaceful protestors which is precisely the reason why people are protesting to begin with.
Rejecting the premises