He seems to be an experienced shooter because the only injuries reported were 2 police officers. All the rest of his victims were killed.
In Boulder, if you were to ask a hundred folks if they carried a firearm on themselves...probably fewer than three would say ‘yes’. In some towns around Texas, it’d probably be near sixty of the shoppers who would carrying a firearm on themselves. That will probably be part of the story as well.
In the video, people are standing around inside the store while the guy is still shooting.
In the age of people glued to their cell phones and/or are walking around high and out of it, it doesn’t take a professional to kill 10 people in a supermarket and being in Boulder makes it even easier.
In Boulder, he can stand there and finish them off without worrying about an armed citizen stopping you.