Voldemort is the only one with a plausible motive to want Bellatrix Black out of Azkaban.
Quirrel drops a bunch of hints directly:
He says he has resolved his parental issues to his satisfaction, and he says they were killed by Voldemort.
After he got what he needed from the Muggle martial arts dojo, Voldemort comes along and destroys it. Later, when discussing the chamber of secrets with Harry, he mentioned that Voldemort would not leave an important source of power lying around for anybody else to use it, so he probably killed Slytherin’s creature.
He tells his whole defense class that he used to want to be a dark Lord.
He tried to become the ruler of magical England by setting himself up as “David Monroe” against Voldemort. Later, once Harry wants to stop being stuck in Hogwarts, he suggests pretending to be Voldemort to set Harry up as a hero everyone else depends on.
He doesn’t want Dumbledore to know whom he really is.
At the end of the Azkaban arc, it turns out that is a very large number of identities, so it’s not particularly implausible to think that he is Voldemort too.
In addition to the rot13′d reason:
It’s true in Canon.
Voldemort is the only one with a plausible motive to want Bellatrix Black out of Azkaban.
Quirrel drops a bunch of hints directly:
He says he has resolved his parental issues to his satisfaction, and he says they were killed by Voldemort.
After he got what he needed from the Muggle martial arts dojo, Voldemort comes along and destroys it. Later, when discussing the chamber of secrets with Harry, he mentioned that Voldemort would not leave an important source of power lying around for anybody else to use it, so he probably killed Slytherin’s creature.
He tells his whole defense class that he used to want to be a dark Lord.
He tried to become the ruler of magical England by setting himself up as “David Monroe” against Voldemort. Later, once Harry wants to stop being stuck in Hogwarts, he suggests pretending to be Voldemort to set Harry up as a hero everyone else depends on.
He doesn’t want Dumbledore to know whom he really is.
At the end of the Azkaban arc, it turns out that is a very large number of identities, so it’s not particularly implausible to think that he is Voldemort too.
Also, mysterious feeling of doom. And Quirrel can sense Harry’s feelings. And their magic can’t interact.