A 2-1 win for the Bees, secured by an injury time goal from Philipp Hofmann, came despite the hosts being reduced to ten men at Griffin Park.
Henri Lansbury had brought the Reds level in the wake of Harlee Dean's red card.
"In the first half we were in complete control and should have been two or three up," said Freedman afterwards.
"We had four or five chances but didn't take them and I'm fed up of saying that after games.
"After the sending-off they were more compact and showed more fighting spirit, while we just got more and more naive."