1. keep it until next feeding day
2. try to return it to the store
3. kill and discard
Personally when feeding live I will put the rodent in with the snake (watching the entire time the mouse is in the snakes cage) if the snake doesn't show any interest after 10 minutes I will kill the rodent then place the dead rodent in the snakes cage overnight.
(If you are feeding the snake in a separate container I suggest that you stop doing this as it can stress the snake out)
And how would you suggest killing it? I would feel terrible but the way I'm seeing it - The snake is my pet and to survive it's gotta eat, the mouse isnt a pet, it's food. Alright perfect, thanks.