There was no smoke monster in the 70's that we could see. If that's the case it would have come after the losties were there. if you listen closely the sounds that come from smoky originate with the experiences of the losties.
Example: John Lock's calculator thingy from when he worked in the box company. AND (some may disagree) when the helicopter carrying Jack, Kate, Aaron, etc... crashes into the water (after the island disappears)- that sound, the sound of the crash is almost identical to smoky's big crashing sound.
So maybe the smoke monster came to be when the bomb was set off, and is somehow made up of the losties. After all who better to judge Ben then those whose lives were made miserable by him.