A hornet sting is usually quite painful, but then it depends on many factors, such as people tolerance, the amount of venom release etc. Based on personal experience it is way more painful than a common wasp sting. The damage that it can cause mainly depends on whether the victim is allergic or not. In case she/he is, sever allergic reactions can occur, possibly leading to death. If not allergic, an hornet sting is not dangerous, while being stung many times might indeed cause severe reactions even if not allergic.