Armed citizens can be effective against terrorists, and people intent on criminal acts, but not much works on crazies.
Anyone with a reasonable amount of practice can un-holster and fire their weapon is a surprisingly short period of time.
"It was "successful" because it was unexpected. "
As opposed to the expected random shooter. Again, I'm quite sure those security guards in the DC Jewish museum weren't expecting to shoot a man dead when they went to work that day, but that is why you train.
Despite your protestations, this man operated in an environment where he knew he wouldn't be challenged because law-abiding people were all unarmed. He was the de facto apex predator today. Had he been seeking victims in a environment full of people who could shoot back, he would have been stopped, even if shooting from a moving vehicle.
I would agree that concealed carry wouldn’t do much in this case as it is really hard to stop a drive by shooting of this type. So yes all the people beating their chests and saying it would be different in the US are not thinking, just look at what happens in the hood, most gang bangers are armed AND expecting a drive-by to happen but rarely have time to react, other than diving for cover. Certainly in rural UK (or in most of the rural US) you are not expecting something like this to happen and even if armed would not be able to react.
The real question is what caused this guy to go postal? My bet is he was on some sort of anti-depressant or Prozac type drug or just going off of it. These drugs ARE more dangerous than a loaded gun because of these types if reactions. Most mass shooters in the US were on or going off of these type of drugs. Unfortunately there will be no call to ban the drugs but most likely a call to ban shotguns will definitely be heard after this.