The initial reaction by Smith is not the problem. He knows the ball was not caught and reacts accordingly. The issue is when he looks back at Wims and sees the player writhing on the ground in pain. Smith doesn't just walk away and maintain some composure. Instead, the Notre Dame product offers up a throat-slashing gesture.
Injuries happen in every football game. The proper response is not to celebrate in the face of a player who is clearly in a ton of pain. The move looked even worse for Smith after what happened on the following play:
Smith had a shot at an interception but had the ball taken away by Allen Robinson. Just an all-around disaster of a drive for Smith and the Cowboys defense.
Smith will likely be asked about his throat-slash move postgame and he should have to answer for it. There is no place for that in an already violent game. Hopefully it was purely circumstantial, and Smith didn't know Wims had been injured.