Okay a stupider theory, but one that is hard to deny
After all the Ushiromiya family is one that faked people being dead to mess up with a girl who nearly drowned mere hours ago even tho she’s a bitch (Erika in arc6).
This is factual.
What kind of family is the Ushiromiya family?
So my theory now is that everyone put “roles” (such as, victim of the first twilight, or milf who gets to wave a gun around while screaming at people) on pieces of paper in a box and had everyone picking up a role.
Then they enacted a very elaborate fake murder scheme.
The reason why the story cuts off at midnight after october 5 is because after that Battler figures it out. “What the hell is wrong with you people”.
He decides his crazy family needs to be obliterated for the good of everyone else.
He sets up the bomb, tries to run away using the backdoor (Kuwadorian). Eva tried to follow him but he escaped first.
BOOOM
The seagulls became fried. UminekoTempura.
A family of crazy people who’d fake death to mess up with girls who nearly drowned hours ago was removed from being a danger to everyone else.
Happy ending.
Also I forgot to add this, but Yasu is completely innocent. Even the Shkanontrice playing multiple roles at once thing was forced on her by others. That’s why Battler tried to save her alone (from his crazy family) but she still felt guilty about participating even if against her own will.