I've seen chickens die during routine wound or illness care just like that (although not during an injection)...they were sicker than they looked and the stress sent them over the edge. I'm so sorry about your duck.
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Did you verify you were not in a vein? An air embolism is my first thought
Did you pull the syringe back a little, before pushing the fluid in? (Ie to check you weren't in a vein etc? ) And if so, did you see anything come into the syringe?
Did your vet go through the injection procedure to your satisfaction with you before sending you home with IM meds?
Sorry this happened to you, try and remember that it was a simple accident, not your fault, you were doing your best for her, somethings, sh-tty things just happen.
Meryl isnt suffering, she's in heaven now, swimming and flying (I believe in heaven, domestic ducks can fly like wild mallards) and living a wonderful life in duck paradise...