All storms are potential killers and it is those aspects of a storm that are not part of the definition of a severe thunderstorm that are the greatest killers. A severe thunderstorm defined by the NWS has hail of 3/4" or greater and winds of 58 mph or greater. Convective wind gusts and hail however are not the big killers. Hail and wind however are responsible for a large degree of the physical damage done to buildings and agriculture.
It is lightning and flooding that are the greatest killers. It is critically important to not be outside when lightning is occurring. When storms occur, it is important to be on higher ground. The following links have safety advice that protect lives: