This is another way to try:
Put maggots in to a large bowl and add cold water to just cover them. Then whilst stirring maggots with you finger, start to add boiling water from a kettle slowly until it becomes too hot for your finger to keep on stirring and just wait until all maggots stop wriggling.
This will kill the maggots slowly and not cause them to stretch or discolour.
Drain off water once all have stopped moving and bag them in an air tight bag and chuck in the fridge.