Any or all of these. Another possibility is the way he did it. He just grabbed the chair and delivered the ***-whipping. No hesitation. No woofing beforehand. You could see the robber freeze for a moment as his brain tried to readjust to the new circumstances. By the time scumbag came up with a plan he was well into a lesson on energy transfer in semi-elastic collisions.
"I've got a gun.
I'm pointing the gun at people.
Everyone is doing what I think they should.
There's a guy in my field of view.
He's not cowering.
He's holding a chair.
Huh?"
*WHAM* *WHAM* *WHAM*
"I'm being hit with a chair.
Searching for backup plan."
*CRUNCH*
*SPLAT*