Actually, wasn't the first Fallout a "set in stone" story? Evil or good, you still have to defeat the Super Mutant Army, and kill the Master. Still comes to the same ending, just depends on how bad you were as to if you kill the overseerer.
Actually no you could join the Master, that ended with the vault being destroyed and you turned into a Super Mutant. It wasn't much of an ending but it was there. Actually don't you shoot the Overseer no matter what? I mean he had it coming lol