I think the Fernsfield massacre is actually one of the most important pieces of background lore, and i'll forever mourn we didn't get to hear more about it in-series
because the thing about vsmp is that its full of characters who've caused mass amounts of death. Scott, Owen, Legundo, Cleo, Abolish
They all have varying reactions to it. Some feel guilt, others decidely do not
Scott doesn't care at all and wants to do more violence for fun, Owen feels justified and like he's right to murder more people. Abolish's job is violence, and he won't think twice.
Cleo feels horribly guilty despite not having any say in actually killing anyone, and Legundo has spent decades attempting to help as many people as he can because he has to try, even after all of his sins
Apo hasn't killed anyone, further distancing her from other characters like Pearl, Avid and (kind of) Pyro
But Apo's seen death. So much death.
It's hard to understand the context and depth of the atrocities these characters have committed. For most of them, all we have are numbers. Hopes and dreams and livelihoods reduced to digits.
For characters like Shelby, these are distant horrors. Owen can look her in the eyes and talk about the slaughter of murderers and innocents alike, but Shelby can ignore it till someone she knows is dead. Only then does it become real, and it doesn't last.
We don't know how well she knew all of the people who died, but considering they were likely her unit, she must have ate, slept, trained and worked alongside them for months to years. They were her friends, her community! She heard them talk about family back home, about dreams for the future, watched them open letters from childhood sweethearts. They did the same for her.
And if you know anything about Apo, you know how deeply they cared for the people of Oakhurst. Caring is in Apo's nature, no matter how much grief and pain and trauma.
Apo is a living embodiment of the faceless victims so many of vsmp's beloved cast members left behind.