I think that Barret's role in the construction of the reactor isn't as important as his role later. When Shinra came and wiped out the town, Barret was absent. He had helped to bring the disaster (like all the others, he supported the building of the reactor). Yet he wasn't there to suffer the consequences. People always get upset when others escape trouble. He was just a scapegoat, and he feels guilty himself for not being there for the town.