One thing you're missing is that this isn't just calling the police. The person in the article phoned the police and described an armed hostage situation. These

One thing you're missing is that this isn't just calling the police. The person in the article phoned the police and described an armed hostage situation. These calls aren't just 'X is committing a crime.' They describe a compelling high risk situation to the police. It's totally understandable that the Police go to the call primed to deal with what they're told is going on: a possible armed perpetrator with the intention to kill people. The police have to go into that situation ready and there's a natural opposition between being ready to react quickly and the risk you react too quickly and mistakenly hurt someone. The other side is the police are risking going in, being too slow to react and getting shot themselves. So it's a very difficult job to do and things WILL go wrong. When you convince someone to go into a house armed and prepared to kill someone, you have to take responsibility that there's a risk someone in that house gets shot by accident.