The police have no idea what limitations they may be facing in the person they’re dealing with. The guy could have been deaf, or disoriented for some unknown reason beyond his control. This wasn’t a case where a cop pursuing a suspect alone had to deal with someone in a darkened hallway, or a perp wrestling for her gun.
They also have no idea how many innocent people will die if they err on the side of caution and don’t deal effectively with someone who is a threat. That is why it is an inherently difficult decision under any circumstances. And it is also why the judicial process should factor in the fact that her primary motivation behind her actions was probably to protect the innocent (including herself). In other words, a very difficult decision, made under very difficult circumstances, with, in all likelihood, a noble and righteous intent.
People who are “limited” need to take extra care in many situations. A deaf person, for example, needs to be extra careful when crossing the street. They need to be extra careful at sporting events. They need to be extra careful when operating machinery. And, they need to be extra careful when interacting with the police.