I'm not sure the Ofglen's were killed because they thought they were complicit in a useful way.
More as a lesson to everybody else to keep their household in order.
They were complicit in that they didn't know what their handmaid was up to and didn't put a stop to it.
So they killed the entire household as a way to install fear into others, to make them more watchful of the others in their house, and to make sure that people were informing on each other for fear of ending up the same way.
They can't have a Commander unaware that a 'terrorist' lives in his house. It makes him weak, it makes him foolish, it makes a mockery of the entire "under his eye" nonsense. The master of the house was unaware, and this is unforgivable to Gilead.
They don't think for one minute that Commander Glen had any useful information on Mayday or that he was part of the resistance. They punished him for not knowing and dressed it up as him being a traitor so he was a warning to everybody else to be more vigilant in their own homes.