These two “hot tags” would be a good place to start researching: wings and disease.
https://www.monarch.org.nz/forum/topic-tag/disease/
https://www.monarch.org.nz/forum/topic-tag/wings/
One of them takes you to a good site with pictures of “problems” which might help you to identify the culprit. I am having about 15-20 butterflies emerge each day but maybe 2 or 3 a week are deformed for some reason. Low numbers – and some will be because they have fallen when drying their wings and couldn’t climb up again – so probably nothing to be overly concerned about here. I have also found 3-4 caterpillars have “failed to thrive” – they just look as thought they’ve given up. Once again, because of the reasonably high numbers in my butterfly house and my own busyness and lack of time, I’m not too concerned. Compared with other animals mortality rates it’s probably pretty average. Can’t expect every one to survive metamorphosis.
We have had quite a few that are hatching out at the moment that have deformed wings. I would like to know why this is happening.
I had one like that and her wings took 5 days to dry & move
properly before she finally flew away north.
Euthanasia has been discussed many times in here – people tend to have their personal preferences. You can put it in the freezer, that’s one way.