I'd discreetly swim to her under water without being noticed. When she got distracted, I'd grab her from behind and force her head under water. She would try to struggle but I would keep her under water. I'd drown her. It would be long and painful. Once I'm done, I'd just let her corpse there.

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Here is Aoi. She's a cute classmate. You've been following her for a few days to snuff her for your personnal enjoyment. When she got back from school she went through a small isolate street. You managed to get behind her discreetly. She has no clue you're behind her. She's at your mercy. You have a knife, a piano cord, a syringe full of drugs, chloroform and a tissue, and of course, your bare hands. There are some interesting stuff in this street, as a hammer, a sack (that could be used to asphyxiate her to death and God knows what in the trash. The choice is yours. FINISH HER.