Q. Why should I disconnect the negative side of the battery first?
A. The negative side of the battery circuit is already grounded, just like you are, standing there… on the ground. Removing the negative cable first breaks the circuit safely. Removing the positive cable first increases the risk that a slip of the wrench might contact the positive terminal on the battery, sending all that electricity straight into your arm. In short, you become the negative terminal. Not pleasant (ask me how I know this).