Okay, first, because even warning shots land someplace. Whose living room would you like it to land in? Second, because she was in the middle of stabbing someone, so there wasn't a lot of time for warning people.
"Why didn't he shoot her in the leg?"
Oh, man, when are we going to rid people of the damfool notion that it's possible to shoot someone a little bit? A bullet in the knee can leave someone crippled for life, possibly resulting in amputation. A bullet in the thigh (or, like I unbelievably heard someone suggest, the "butt") can sever the femoral artery and cause them to bleed to death right there on the spot. Also, she was in the middle of stabbing somebody, so there wasn't a lot of time to play eeny-meeny-miney-moe with bullet placement.
"Why didn't he deescalate?"
The time to deescalate someone is before they start stabbing people. Once they're actively trying to stick a knife in someone, which is lethal force, it is generally accepted that you may use drastic measures before they can finish killing that person.
I don't know everything that led up to the shooting of that girl. Supposedly she was the one who called the cops. Maybe she got tired of waiting and decided to take matters into her own hands? There's also the phenomenon where people call the police and automatically think that it activates the "I'm the Good Guy" light over their head, that the cops will show up, divine that they are the injured party, and back their play. This is how the occasional homeowner gets shot by responding officers when they wander out looking for the burglar with a gun in their hand.
There's a lot here we don't know, and it sure seems crappy all around, but an LEO shooting Person A to stop them in the act of using clearly visible lethal force against Person B is usually pretty cut and dried.
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