For me the two that are questionable are not noticing or questioning the unexpected door mat and not asking why the door was ajar, although upon noticing the door she might have immediately jumped to the conclusion there was an intruder in her suite. (More on this in the next paragraph.) When you have no reason to suspect you're on the wrong floor there's no need to look at the suite numbers, lit up or not. On my walk from the elevator to my apartment I'm often fiddling with keys, trying to get the door key ready to insert into the lock. So my attention is focused on the keys. I can see the walls and doors in my peripheral vision so I know how many I've passed, but I certainly don't notice the apartment numbers on the doors.
Are you familiar with the idea that the human brain has two separate thought processes? One is very fast and jumps to conclusions, allowing for quick action in the case of a threat. It runs on a hair trigger and often makes mistakes, but can be life saving in an emergency. The second system is the analytical one, but it's slower to start up and takes longer to come to conclusions. I suggest that upon seeing "her" door was ajar (although that means she ignored the unexpected door mat) the fast system leapt to the conclusion "intruder!" And there was one, so she took quick and decisive action against it. Only then did the slower system kick in and start to really analyse what was going on. (Oh boy ... I can just see JoeMorgue saying "fan fiction!" And, Darat, based on this idea it was her "fast" system that governed the sequence of draw gun, aim, fire, aim again and fire again. It's possible her police training helped her with that—although I there's no way I can say that for certain because I don't know what her department's training procedures are.)
Having said that, there was no reason for her to draw a gun and fire at an unexpected person. Drawing the gun on what she likely thought was a burglar is likely to be expected, but as others have pointed out she should have immediately retreated. Had she done so there would have been some words and apologies exchanged, and life for both of them would go on.
At the very least she's guilty of whatever Texas law uses for "manslaughter."