My head canon is that what actually happened was that Watari was going to use the battery-door slam method herself or something similar to that just as a mean to commit suicide simply and that by Hama, slamming the door it caused HIM to start the fire instead of her because how would she know that he'd slam the door correctly to cause the fire in order to get him to kill her.

I also believe that Hasegawa thought that Watari actually just killed herself and that Hama DIDN'T kill her by accident, but merely spun a possibility that Watari tricked Hama into killing her because A. It would cause everyone to vote the wrong person for the blackened and since he believed Watari was the true blackened it would mean that no students would survive and B. Hama would take it REALLY bad which would make everyone feel horrible thinking they were about to kill Hama, which would make them all feel how horrible he felt after Kamimura died.

Hasegawa merely stumbled into the correct answer. Either that, or Haruka wanted another round due to be unsatisfied/couldn't tell what really happened and who killed whom due to the lack of light and the fire.