Oh no! Well that's just the way it goes sometimes, I wouldn't beat yourself up over it. I just recently killed a developing egg because I set it later than the rest of a batch of eggs and it had to go through the high humidity of lockdown before it was due to hatch. I thought I could maybe get away with it but it ended up dying in the last few days of incubation and when I opened it up there was liquid in the air cell which is a sign of the humidity being too high for too long.The remaining egg could still make it, you just never know. I don't know of anything further you could do but just wait and see what happens. If the chick hasn't lost enough water weight it may have a difficult time hatching so I would consider assisting it if it looks like it's having trouble. Just keep in mind that assisting can do more damage than good if you don't give the chick time to absorb the yolk sack and the blood in the allantois.