Optimus didn't want to put him in too much pain. Enough to get the point across that Sentinel had made one powerful enemy in what he did. From Shockwave to attempting to radiate those sparks...sending the Elite Guard after him, there's plenty of reasons he wanted to do this.

He watched him scoot away, eyes narrowing as he speaks up again. Justice? Not quite. Granted it was probably deserved, not in the physical pain method, but...

"No. That was just expressing my opinion of the situation. That was from Orion Pax. Personally."

Just try it, man. We'll see who comes out of this alive.