I'm not really sure..She may be playing and she could have watched it on tv or something and that gave her the idea. But I would talk to her and ask her why she is doing that. Dig deep.
Why do you hang your animals? Were they being naughty? etc...Maybe you can figure out if she is just playing, and mention casually ya its not okay that we be mean, only pretend play..
Well... children do what they do. I should know; I am one. Basically, your child has an outstanding imagination, and I do pretty much the same thing when a stuffed animal of mine robs a fake bank or something. I wouldn't worry. She may even be just helping them to "fly"
Is she hanging them for Decoration purpose or is she "killing" them off in a game! Two different things.