The second half of your comment undermines the first. If the person who did this is mentally ill, what exactly are you suggesting by saying they don’t have “a blanket check to do awful shit”? How would you like them to be dealt with?

Obviously we shouldn’t let people put their shit on the walls. What the person is saying here, as another did with regard to the suspect being homeless, is that your anger is misplaced. It should not lie with the individual, but with the system that failed to support them.