It's nice to actually get to see the heroines' perspective of the event.
The reason for their guilt here is very interesting. It's not just that they felt powerless during the fight with the reaper, it's also that they thought it was beautiful. That moment where their companion almost died and...
Well excuse me for responding to a oneshot filled with flowery wordplay with some of my own because I genuinely love the story of those two and appreciate their happy ending. I'll try to sound more robotic next time just for you since you seem unfamiliar with genuine passion.
In all their struggles to save others, they were given little rewards, but that's okay. It led them to each other, and so they struggled more until they eventually saved each other. This is their true reward, their peace, their love. Avalon.