Personally I’m assuming that Quirrelmort rescued Bellatrix to influence Harry. He now has first-hand evidence that Harry was engaged in a major crime, has shown Harry a part of the Wizarding world which he could never accept which was enabled by democracy, has shown Harry that Dumbledore isn’t as proactive as Harry would want him to be, almost convinced Harry that a revolution is necessary, convinced Harry to lie to protect him, and has made Harry feel sorry for Bellatrix. Generating a ‘false alarm’ of Voldemort’s return is a nice side effect as well if he doesn’t plan on acting immediately, Dumbledore may well have just expended significant political capital too early. If he was planning on going through with the ritual, then he had Harry right there with him. (Although he could have just taken some blood while Harry was asleep, of course.)
The only motivation I can see for him to retrieve her otherwise is that he couldn’t remember what happened at Godric’s Hollow or to the Longbottoms, so he wanted to retrieve Bellatrix to find out, he regretted what he did to her (seems unlikely), or he’s not Quirrelmort, and is really just a powerful but neutral wizard who wants to try and retrieve some of Slytherin’s magics.
The most obvious motivation given what we know about Quirrelmort and what he needs to regain his old power is that he retrieved her for use in the spell that will restore his body. For his motive to be anything else would constitute a twist.
Personally I’m assuming that Quirrelmort rescued Bellatrix to influence Harry. He now has first-hand evidence that Harry was engaged in a major crime, has shown Harry a part of the Wizarding world which he could never accept which was enabled by democracy, has shown Harry that Dumbledore isn’t as proactive as Harry would want him to be, almost convinced Harry that a revolution is necessary, convinced Harry to lie to protect him, and has made Harry feel sorry for Bellatrix. Generating a ‘false alarm’ of Voldemort’s return is a nice side effect as well if he doesn’t plan on acting immediately, Dumbledore may well have just expended significant political capital too early. If he was planning on going through with the ritual, then he had Harry right there with him. (Although he could have just taken some blood while Harry was asleep, of course.)
The only motivation I can see for him to retrieve her otherwise is that he couldn’t remember what happened at Godric’s Hollow or to the Longbottoms, so he wanted to retrieve Bellatrix to find out, he regretted what he did to her (seems unlikely), or he’s not Quirrelmort, and is really just a powerful but neutral wizard who wants to try and retrieve some of Slytherin’s magics.
The most obvious motivation given what we know about Quirrelmort and what he needs to regain his old power is that he retrieved her for use in the spell that will restore his body. For his motive to be anything else would constitute a twist.