This is a very interesting thought: the Dowdy alien you mention has wiped out an entire race of 50 billion beings, but is aware of his crime and lives in self exile. He's committed an atrocious crime in a rage fit.
On the other hand, the changelings committed a planned crime with intent to cause suffering.
I'm not sure who's worse: the Dowd by magnitude of the crime, but the Changelings ruthlessly command their Dominion.
I dont remember the name of the episode, but Bashir took a leave of absence and hung out on this planet for, I'd guess, a year to work on a cure. He didn't find a cure but managed (by accident) to make a vaccine that saved new-borns from getting infected.