@Tsuki09
You're right about the time ( yeah, 30 years was what I remembered, because it was in the beginning of most chapters ). But about the fact that the demon Frieren let live being the one that is causing the issues in the city proper in the present ... you can't really say that she wasn't for killing demons after seeing her looking down on him while in top of a demon bodies pile
.
The fact is that this particular demon is alive and not a part of that pile is solely because she didn't felt like killing him ( or, most likely, because Hero guy said her to not kill him because he wasn't in the way at the moment. I would bet we will see someting about that in the next chapters ) for whatever reason. And I'm pretty sure that a good part of what he devised in the last decades were in response to having seen Elf Wizard, the Demon Slayer , in action
So yeah, her actions in the past are having consequences now ...