He FAILED to kill eighty-percent of the people he shot. He took advantage of unarmed people who were evidently indoors at a mass gathering. The typical response to a shooting like this would be to hide behind an object or attempt to use an exit. Neither strategy would diminish the availability of targets to the shooter.
Shooting a gun and hitting a person-sized target at a range of twenty feet is not rocket science. To kill a dozen helpless people in minutes is not that difficult. It was done at Columbine, Virginia Tech, and numerous other places.
The common element is that the potential victims are gathered together in a gun-free-zone. I doubt that this type of attack could occur in Israel. From what I have read, they do not disarm their own innocent people.
Even 13 killed is pretty high for 3 minutes.
You have to factor in adrenaline as well and people moving as well.
I still don't think we are getting the entire story, but it does look like he did accomplish it by himself.