Why is being struck by lightening bad?

Well inside your body, the main way for communication from the brain to the body are by electrical signals (from the neurons). With a jolt of electricity, this disrupts the signal. (think of it as lightning striking an electrical wire.. there is a surge of electricity, and without a surge protector... well your screwed). Also, inside the heart, electrical signals are used to control you heart beat. Mess around with this and you could get a heart attack. Plus you won't always die.. you have a high chance though.