Anyone who comes to the aid of another person can find himself in this situation... The attacker charges straight at you, thereby putting his original victim in the line of fire, in the background. What do you do? Just asking.
I think the young lady did the right thing, but don't know for certain; she also definitely put her aunt in danger of being shot as soon as she opened fire. I'd think the girl could've moved sideways as he charged or gotten down low to the ground to increase the angle between her aunt and the attacker... But in that case she'd be a shooter who was in motion and more likely to miss her shot... So I just don't know... Just asking.
Anyhow... All's well that ends well.
What was the biggest danger to the aunt, the niece shooting the perp dead or letting the perp kill the niece and the Aunt? The niece did the right thing. As far as her “moving to the side or dropping down”, you obviously have no idea how fast these things move. The perp was just feet away from her and she had no time to think of such things, she just acted, and in the correct manner. As for hitting her Aunt, she probably needs more range time under pressure to improve her shooting skills.
Nobody does, but likely you are right.
A likely fatal attack was in progress, so acquiring and preparing superior force (the weapon) was reasonable. The attack was continued so force to terminate the attack and prevent the attacker from acquiring the weapon was also reasonable.
Likely the young lady fired from the best position available, in a manner most likely to hit her target (standing still), and discontinued fire prudently.
All in all, very good decisions under extreme pressure.
Some of that depends on the caliber of the weapon (not listed). If it was a .45, it would have gone right through the guy into whoever was behind him. A .22 probably wouldn’t have killed him with two shots, unless they were in the right spot. A .380, not full metal jacket, better chance of putting the target down, without going through, big mess for the coroner though (but, really, who cares, he’s dead, yea!).