If the flash actually existed and could run at the speed of light, he would run so fast that instead of the air being pushed out of the way when running, the atoms of the air would not have enough time and fuse with the flash.
Which would create many gamma rays, x-rays, Creating a giant plasma bubble/fireball type thing.
That would cause the atoms from the flash to scatter everywhere at about the speed of light, hitting and fusing with other atoms making more x-rays and gamma rays
this would make a huge nuclear explosion expanding miles.
So in short the flash would blow up himself and thousands of other people(Assuming he is in a city),
But if superman flew at the speed of light, he would be fine, but he would end up being the supervillain, since it would blow up thousands of innocent civilians