Ed is possibly the first person sent outside the Vault before the water chip broke. Considering he never made it very far (about 2 and a half steps from the vault door), he was probably killed by the mutant rats that inhabit the cave outside of the vault.
A quote about Ed by Fallout lead designer Chris Taylor: "What's the deal with Ed? Ed's dead, baby, Ed's dead." This references a line from the Quentin Tarantino film Pulp Fiction, spoken by Bruce Willis' character, Butch, differing by a single letter (Zed's Dead).