Eobard Thawne aka Professor Zoom/Reverse Flash is a terrifying villain. Think of a man who can travel through time and manipulate everything that happens to you. He can make your life a living hell and you wouldn’t be able to do anything, or know it was happening. He could literally pop up anywhere at any time. That is Reverse Flash. The Flash had to break his neck to stop him from murdering his wife after he made him experience it already.