Why are traitors considered by Dante to be the most evil doers of all?

The traitors are distinguished from the "merely" fraudulent in that their acts involve betraying a special relationship of some kind. There are four concentric zones (or "rounds") of traitors, corresponding, in order of seriousness, to betrayal of family ties, betrayal of community ties, betrayal of guests, and betrayal of liege lords. In contrast to the popular image of Hell as fiery, the traitors are frozen in a lake of ice known as Cocytus, with each group encased in ice to progressively greater depths. Traitors are not merely flaws of desire but spiritual flaws that go to the core of the human soul.

Fraud which seems to be close to what you mean in the 9th circle, alludes to the very corruption of the devine soul. He positioning of fraud as the worst of sins helps us to define evil: fraud, more than any other crime, acts contrary to God’s greatest gift to mankind—love.