If the guy wasn't in uniform, then this is a tragic accident and nothing more.
shut the fuck up
No
You should, it is not an accident when you willingly pull the trigger an kill someone. That is a murder. If someone in your family would gunned down like that would be ok with it ?
Mistaking an armed criminal for the armed man who subdued and kept a gun trained on him is absolutely an accident. It doesn't matter that it resulted in the security guard's tragic death. And the question of whether or not it was obvious he was the good guy is important in determining how much negligence should be attributed to the police in this situation.
Also, you seem to have this bizarre idea that accident means okay. The two words aren't related in any capacity. That's why I called it a tragic accident.
There's no such thing as an accident involving guns. Only negligence.
They aren't mutually exclusive. In fact, they very frequently occur together.