the proliferation of "shooter" computer games ( which desensitises the action)
Links to video games have been debunked so many times now.
These kind of incidents are simply going to repeat constantly as there are always madmen / criminals around and the easily availability of guns means they can kill a lot of people in a short time and with the police being armed , they often guarantee their own death in a macabre murder / suicide death wish. Sadly it is something Americans just have to accept , and , in fact , incidents like this just increase the chance of copycat events.
I agree that the TV media and also social media have made things worse and I believe are now some of the biggest additional drivers for the epidemic of shootings.
As for the police, I saw an article that had analyzed shootings and apparently the number of people killed before a shooter is shot by police is around 14 but the number killed before they are shot by an armed citizen is around 2.
It appears in this case the police were already engaging but them sat back for reinforcements to arrive.
If the plan is for everyone to be unarmed and have to wait for the police ... well, it's a bad plan. An example of how stopping law-abiding citizens from having guns could make things worse.