I can definitely see where you’re getting that sense from. To me it seems less like sadism and more a natural, hungry curiosity and fascination with a world that’s been withheld or distanced from her for her whole life. Plus she’s a 14 yr old teen, they tend to get excited about the idea of dangerous, forbidden things like guns or knives lol so that part reads very typical teen to me.

The moment with the infected she found in that basement is what definitely feels a bit concerning, and the only thing I’d say felt as straight up sadistic so far. But it could also be pretty easily explained as a fascination with something that has had such a big impact on her life and that has taken away so many things she views as wonderful aspects of life before - like the airplane she and Joel see. And it’s something most people don’t really get an opportunity to see up close with little chance of danger. So she hurts it out of malice, or to see if it does hurt, or just to see what would happen.

I’m not too worried yet but who knows lol.