Oh, another hypothesis, based on something I’d forgotten: Slytherin’s Monster was supposed to be keyed into the wards at a higher level than the Headmaster himself. And in this version, it had knowledge it transmitted to Voldemort. What if that included knowledge of how to hack the wards? And then Quirrellmort decided to make the wards register himself as the killer, as a purloined-letter type deal?
Oh, another hypothesis, based on something I’d forgotten: Slytherin’s Monster was supposed to be keyed into the wards at a higher level than the Headmaster himself. And in this version, it had knowledge it transmitted to Voldemort. What if that included knowledge of how to hack the wards? And then Quirrellmort decided to make the wards register himself as the killer, as a purloined-letter type deal?