“Now, shall we begin!” he roared, not as a question but as a way to rile up the awaiting fiends.
Even if he's just whipping up the crowd, it's still a question, and thus REQUIRES A QUESTION MARK. If you just put their reaction right after "he roared," as in "…he roared, and their answering cries of 'sacrifice him!' were loud enough to shake dust from the ceiling."
