The window where deadly force could be legally used and positively affect the outcome (i.e. stop someone from dropping a rock onto car on the highway) would be pretty narrow, but it certainly could exist -- looks like it did in the story linked in the OP.
Consider this from the story (emphasis added):
“We were on our way to Fort Worth and traffic was backed up. I saw the guy on the bridge in front of us and he had a rock over his head,” he said. “I saw him throw it and hit one vehicle. Then he hit the one in frontof us. He had it over his head and I pointed at him to tell him not to do it.Obviously he done it anyway.”
“The rocks landed inside her vehicle. Praise God it didn’t go through on her side because it probably would have killed her,” he said.
So the cars were stopped, and he could plainly see the guy throwing rocks. The rocks were big enough and dropped/thrown hard enough to break through the windshields. He could see the guy ready to toss another one -- there was time to do something about it. What to do? Wagging a finger and verbally challenging didnt' work.