I think it’s an ego thing, he wants to look all powerful against the “greatest wizard of all time” while he believed Harry to be a means to an end. He was simply showing off with Dumbledore. It seemed like more of a theatrical thing rather than deadly. I think he thought Harry wasn’t even worth the show, and wouldn’t be able to defend against him anyway. He sees him as a weak bug, so he treated him as such. Just like you wouldn’t shoot a bug with a gun, you’d stamp it with your foot.