"We are agreed" means we have a consensus, something everyone agrees with.
The leader of a committee that seems to have come to a decision may ask the group, "Are we agreed?" If the answer is yes, everyone agrees, then the decision has been made. If someone disagrees, then they may continue discussing until everyone is satisfied.
The writer has chosen this particular phrase for the humorous quality that it rhymes with "greed". He could have said "Everyone agrees that greed is bad," and it would have had the same meaning, but not the same effect.