Do you think it is a harmless prank to mess with another persons account? What would you do if you find a computer was left on and accounts were not logged out? What would you want someone to do if you left your computer on and accounts open?

No, It is not harmless to prank/ hack someone else account why would you want to do something like to hurt another person.  If a computer was left on I would automatically log out because its not my account.  I would want somebody to log out my account if I had left it logged in, I hope they would have the benevolence to log out.