Why can there ever be only one motivation for fighting? It's certainly possible that both could have motivated them, or at least the trio since Barret isn't to that point yet. But by Jessie's words, the others are. They're at least aware of what they've done, and by the remorse she shows, it's not a stretch to believe that part of the trio's motivation for fighting in the pillar could have been to atone for what they did as well as protect their home. One reason does not cancel out the other.
And then your potential headcanon reason.
And Jessie's words say nothing about Biggs or Wedge, and doesn't even explicitly say she's feeling remorse, much less that it was the whole reason she fought. She can say (depending on what Cloud says), that they killed a lot of people, so maybe her dying is punishment for that. But that doesn't really say anything about WHY she fought in the first place.
The point is you make declarative headcanon statements which could be the case but are by no means obvious, and then accuse others of willfully missing these things.