Okay, I watched the video. Here are a few quick things I picked up in the video...
The gunman "retreated" multiple times between rooms and out of the building, only to return and start anew. People talk about how terrible it is to shoot bad guys who are fleeing, but this is a classic example of how "flight" is determined only AFTER the gunman has actually left. Just because the gunman is going away doesn't mean the fight is over. On his last time leaving, he went after a woman and killed her. Then went to his car for a 2nd time and went to leave. Even after he took flight in his car, having left the mosque, the danger was not over and he shot (at) people from his car.
He was NOT disarmed as claimed in numerous reports, but I think the guy did try to take him down. He failed.
Nobody tried to exit the windows. Building only had one way in and out??? When people did try to take flight, it was out the front door only, it appeared.
Gunman did NOT search one large side room or bathrooms by the large room. He took only easy targets of opportunity. He did not search the side room on the other side, but could see into most of it as he passed by. I suspect survivors from the mosque that were unshot were hidden or out of view in the bathrooms or side rooms that were not searched.
People huddled together long before they were shot which made for easy, stationary targets.
He repeatedly shot dead people or people down, or down and moving or making noise. Playing dead that has worked in numerous other mass shootings would not have worked well here. I may have miscounted, by some people who were down were repeatedly shot as many as 5 separate times. A few may have been shot even more.
Passersby did nothing.
He had a reload-related malfunction (?), had trouble clearing it and did not take cover at the time. He stood out in the open, in view of the open front door, in the middle of the largest room.
I don't think he was well trained or necessarily even trained at all. He just had easy targets that essentially waited to be shot in many cases. That he repeatedly shot people seems to be what drove up the death rate. He certainly wasn't unfamiliar with his weapons, but not to the point to indicate that he was highly trained. Mag changes were not smooth, the malfunction clearing wasn't smooth, he dropped a fully loaded mag and never retrieved it (explained by him in drive afterwards), he left a gun on the ground, during the shooting, he left his vehicle open and all the spare weapons exposed, etc. etc. etc. His strongest skill seemed to be to pull the trigger a lot.