Killing Hornworms

This is maybe a little off topic, but I don't know who else to ask. The other day I bought some hornworms for my beardie. I was watching them for a day or two before feeding them off just to make sure that they're healthy. A few of them are shriveling and dying, and the others aren't moving around like they usually do (I'm picking them up and I can see their legs are moving a little, so I know they're alive, but they're not wiggling around at all). I've decided that I don't trust feeding them to my beardie, and would rather play it safe than be sorry later. I also can't release them because they are pest insects. Does anyone know of any humane ways to kill them? I was wondering if I would be able to put them in the fridge to make them enter a more state, and then put them in the freezer to kill them? If anyone knows any better methods, please let me know.

(I know it's not technically humane to kill any living creature, but if they're going to die anyways I would rather not let them sit in a container and suffer. They would normally be fed alive to a lizard, so I figure anything has to be at least a little bit better than that.)

Putting them in the freezer is probably the best way you could do it. No need to put in the refrigerator first.

-Brandon