Thunderstorms can be classified as dangerous, for many reasons.
In thunderstorms, many changes to the weather are present. For example, lightning is present. Lightning strikes causes injuries to 240,000 people a year, and cause about an estimated 6,000 deaths worldwide in a year. Lightning can also cause fire to buildings, being a leading cause of barn fires.
There are also other conditions associated with thunderstorms, such as strong winds, tornadoes, and rain/hail (depending on the temperature).
Other conditions, such as floods, power outages, and flash floods have occurred during thunderstorms, flash floods specifically being one of the primary causes of death in a thunderstorm.