I sometimes need to kill them for fishing and i just put them into boiling water.
Before you ask....sometimes i throw quite a few maggots in the water as groundbait and if they're dead, they can't wriggle away from where i want them !!!!!!!!!!
What is the difference between a fly maggot and a cute seal pup? You think it is ok to boil alive hundreds of animals purely for you pastime pleasure?
Before you ask what good a maggot is, if it wasn't for them and many related species this planet may not have animals on it (uncluding us). Can you imagine the diseases that would be rife if all the dead animals would just lay there rotting for ages?