I could see why he was going into this speech and it was a horrible way to go about it. He wanted to put fear into the children who were possibly thinking about joining Voldemort. He believed that by showing a force, he would prevent any of them from joining. Doesn’t he realize that would cause the opposite effect? All this would do was cause them to run to Voldemort. Showing your power, putting fear into someone who didn’t know how to handle it would run straight to the other side.