Is ringworm ever deadly?

No, but . . .: Ringworm (tinea corporis fungal skin infection) is never deadly, but one can never say never. So, it is theoretically possible that ringworm's fungus could invade to the inside of one's body (if he had no functional immune system) and cause an overwhelming internal fungal infection and kill him.

No: Ringworm will not grow at body temperature, which is good. Reptiles (cold-blooded) have been known to die of these fungi. If you were this severely immunocompromised, some other infecton would have carried you off first. By contrast, the acne bacterium occasionally causes deep tissue infections, especially after heart surgery.