"Ele está dizendo que ele já teria se matado se acreditasse nisso?"
Provavelmente não.
Quirrell responde a pergunta de Harry sobre a vida após a morte duas vezes no mesmo capítulo. No começo é uma questão geral:
Professor Quirrell," Harry said suddenly, "is there an afterlife?" The Defense Professor raised his cup to his lips again before answering. His face was thoughtful. "If there is, Mr. Potter," said Professor Quirrell, "then quite a few wizards have wasted a great deal of effort in their searches for immortality." ~Harry Potter and the Methods of Rationality, CH 40
Quando Harry o questiona novamente, com a intenção de descobrir quais são as crenças de Quirrell sobre o assunto, ele recebe a resposta de que Quirrell não estaria no mundo dos vivos se ele acreditasse que há vida depois morte.
Congregar essas duas respostas fornecidas por Quirrell deve responder sua pergunta mesmo sem o spoiler, mas apenas no caso:
Seeing that in MoR (as in the original book Harry Potter and the Philosopher's Stone) Quirrell is possessed by Voldemort who is alive exactly because he was trying to cheat death creating horcuxes, it's more likely he is referring to his own efforts dabbling in necromancy to stay alive than about committing suicide to experience the afterlife. It seems the second answer is a clue to the readers about who hides behind the mask of Quirrell.