News sources are reporting that Expavious Tyrell "Taylor donned a clown mask, stood on the margins of the festival's revelry and barely had an hour to enjoy it before the attack, said his girlfriend, who attended Zombicon with him." And that he wore a "creepy clown mask."
Cory Panshin notes:
I don't find anything about Roman emperors -- but it seems to be related to the Latin expaveo, meaning "I am afraid" or expavi, "I began to fear." The latter has some odd associations.