I think we should all remember that Matalas said that Vadic and her group have sympathetic reasons for doing what they do. As destroying a Starfleet recruitment center and impersonating whole swaths of Starfleet are horrific actions in themselves, for Matalas to make that kind of statement in regards to the changelings means the Federation must have done something really, really, really bad for anyone to remotely sympathize with Vadic and the Changelings.
They tried to genocide the changelings during the dominion war by infecting Odo with the disease.