So Quirrell wanted the Stone; he presumably looked into the Mirror and saw himself using it to resurrect Voldemort. Or possibly further-resurrect Voldemort, if that voice was indeed the disembodied spirit of the not-entirely-deceased dark wizard. (Actually, pointed out the part of Percy's brain that was not currently freaking out over the reminder that Voldemort was possibly in the room with him, that would totally explain the unicorns.) 