That's going to be for a jury to decide, if it ever gets to court (Wal-Mart will settle). It won't take Clarence Darrow to point out that putting someone face down and having several security guards lay on top of them while they turn gray and plead for their life is a pretty clear-cut example of deadly force.
As a matter of fact they would have been justified to run after the guy and hit him in the knees with a ballbat to stop him but instead they wrestled him to the ground and subdued him till the cops got there.
Your analogy does not work. If they had him subdued and then continued to beat him with bats until he died, that would be closer to what actually happened here.
He died while being subdued.
He died after being subbdued. That is an important distinction.
Now you are just making things up.