The whole invasion of St. Curtis arc. Not only does St. Curtis, a previously peaceful town, get destroyed while the demons are hunting angels but you can see the destruction it's causing. Oh, yeah, and the whole place is burning to the ground during that portion. The fact that Kieri leaves the group, Kinako is split up from Tirol, and that angels are revealed to have been working with demons during the invasion doesn't help it any less. Oh, and it's shown that it's not just angels that are getting attacked. The fact that the same thing happened at Weyville just a few arcs ago doesn't help, nor does the fact that it takes place during the Flower Festival.
Lazuli's return in St. Curtis. Killed off one of the series' Ensemble Darkhorses? Within sixty seconds of her intro! Nearly kills the protagonists? Three times, in fact! If it hadn't been for Azurai, of all people, she probably would have killed them too. And that's not bringing up what else she could have done before finding Rhea and Buwaro, or what would have happened if Kieri hadn't shown up when she did.