She even tracks them down in the middle of nowhere. I'm sure they appreciate her going through all that effort.

They better ::3

You would almost think they would hunt bigger, more filing prey.
Humans would be, decoration. The collar seems to agree.

To be fair, I do not think they need to eat at all, as any perceivable form they take is a minuscule projection of the being behind it. Besides, not every food item is made with the sole purpose of filling hunger, i.e. candy, or most other confectioneries for that reason
And who says they can't be both? ::3

Why kill if she doesent need to eat

Because of how my settings are set up, any being in any mortal world is essentially irrelevant, the only thing that matters is a soul that inhabits it. For souls death of their vessels means nothing, they exist outside time and will either reincarnate in another world or go back to the Sea of Color. Seeing how Ohta, the personification of the concept of interaction, was one of the designers behind that system, she is acutely aware of how it all works.

But if we talking about actual reasons, who knows? Maybe her prey is a weirdo like me and wants that? Maybe it's somebody who deserves it? Maybe she's just toying with them. No-one says they are going to die, and most stuff I draw is poorly thought out garbage open to interpretation, unless stated otherwise ::)