The answer seems indeed to be "he was geniality personified".
"Atrocious" - the adjective that originates the adverb "atrociously" - means abominable, dreadful, or even cruel, brutal. On the other hand, geniality is the ability to behave in a cheerful, easy-going way, with affability.
The phrase "he was geniality personified" clarifies the meaning of "atrociously" by providing its opposite.