I’m an LEO, so I’m going to give some personal insight as to why this cop still fucked up rather than just speculate like some of the other comments.

Step 1.) After arriving to a potential burglary, you conduct what’s called a perimeter check around the building in question from afar. This is done so that it gives you a good reactionary gap in the event things were to break bad since you’ll most likely be using available cover/concealment while you do so. It also allows you the opportunity to see if you can see anyone inside. If you happen to see someone, you watch their behavior to determine whether they’re burglarizing the building or not.

Step 2.) If you observe any points of forced entry (including open doors) during the perimeter check, you hold that corner of the building from afar so you can observe two sides of the building. Then you tell another responding unit to hold the opposite side of the building so that they can cover the other two sides of the building you can’t see. Then a minimum of two different officers approach the point of entry, and announce themselves as LEOs so that it gives anyone inside the opportunity to surrender (if they’re burglars)/verify that they’re supposed to be there.

3.) If you conduct then perimeter check from afar without observing anyone/signs of forced entry, you do another perimeter check from up close to the building. This is the most dangerous step of the process, hence the reason why it’s last. You physically inspect each door and window to see if they’re unlocked. If you find one that’s unlocked, you and a partner announce yourselves as LEOs before making entry (same method/reasoning in Step 2). You also look through windows to look for people acting suspiciously/signs of a burglary (items strewn about). It’s really important during this step to not rely on using your flashlight too much. The reason for this is to prevent easy target acquisition from anyone wanting to shoot you from inside, but also to not startle any legal occupant inside since they won’t be able to see you (which also might be armed since they think they’re being burglarized).

Step 4.) If everything’s secure, it’s safe to assume that the call is not in fact a burglary. You then knock on the front door like a normal human being and tell the person inside why you’re there.

With that in mind, the cop is going to try to justify his actions by saying he responded to a potential burglary, which was substantiated by the open door and items strewn about the residence. He’s then going to say while looking through the window, he saw someone that was armed and made a split second decision to shoot. It’ll be interesting to see what the verdict of this case is, but it all could have been avoided if he followed the above steps.