Deciding to put a stranger-USB device in your computer is a thing you do, and decided to do. Getting hit in the head with a bat isn't.

"I'm not a lawyer" etc., but I'm pretty sure that, in many if not most nations with a broadly Western judicial system, the deliberate planting of this device with the intention of causing harm will be illegal. It doesn't matter if the poor fool plugs it in: you knew what would happen. Furthermore it's arguably true that you intended it to happen. Therefore, you're a dick and you're at fault.

No, but it's a great way to increase "don't stick your head where it shouldn't be" awareness!