So let me get this straight.
Fumiko is telling Denji that he should ally himself with the War Devil because the world is already shitty so it would be better to have a shitty world where people are immortal...even though she knows that people turn in to trees as Aging explained.
But more importantly, she is arguing that the War Devil could have already weaponized him if she wanted, but refused to do so because she loves him, so she's telling him to let himself get killed by the War Devil s...they can live together happily.
How any of that makes sense?
>War loves you because she didn't kill you.
>So you should let yourself get killed by war.
>So you can live happily with her.
???