Mukuru was given a list of people to kill. People who bought furniture made out of Austercity children. The people he killed when he was shot with incendiary rounds were innocent, but he was panicking, and in pain due to the flames consuming his body.
Was the kid truly wrong for wanting revenge on the system? His action have consequences, but so did the actions of the rich who ordered a fancy desk made out of his sister's remains. Shouldn't have gotten involved in that kind of business if they truly cared for their precious families.