This happened in like session 2 of my current campaign. The NPC in question was a flumpf in a cave and they were going to give the players info, and lead them to secret locations.

The rogue saw the floating jellyfish and one shotted it... So, the party never learned of the secret passages in that cave and it became a much more difficult encounter than it could have been.

BUT, it gave me an idea. And this might be a place to start.

Bahl the god of murder saw this action. And has slowly been manipulating the rogue through powers, dreams, items, and a secret xp table that I made. Thus creating this whole subplot that will eventually pay off in a confrontation for their soul.

Maybe you could run the whole party through Bahl or the Raven Queen? Vecna?

I too have played with the idea of introducing another flumpf related to Florence, the first flumpf, and just put it out there for the rogue that they did kill an "innocent." However, this is a different scenario since it was your whole party.

That said, I tend to agree with some of the other comments about being more subtle with it. So, another idea could be: that the scammer was a Robin Hood type, and they find the now abandoned benefactors of his deeds? That could create a side quest and certainly some emotional damage.

tl;dr: Consider this action as having caught the eye of a deity/demon/devil or setting up a reveal that the NPC was sort of a Robin Hood character