Duo was definitely strong enough to do something like that. At least Ehmoi seemed happy about it.
Or deluded and biding his time until he could kick Duo's head in.
"I'll assume that was a good thing for him to do." The Duo he knew generally avoided violence unless he had to. If the brother was the one who had gotten Ehmoi into this state, then Trowa could see where Duo would have felt it necessary.