Dear friend, euthanasia is not killing but having the heart to do the right thing even when it's hard.
You did the right thing, I truly believe that!
Also by re-focusing on the babies that came out perfect!
21 days would be more normal.
Since your other chicks are good it could just have been an anomaly. Or maybe from a specific hen if it was more than one.
Check out the link to my favorite hatching resource. Reasons for embryonic mortality and other hatching issues starts on page 51..
https://www.hubbardbreeders.com/media/incubation_guideen__053407700_1525_26062017.pdf