Giving them the herb isn't going to keep them from panicking. It is quite possible to panic while sedated, I have seen plenty of animals do so even with heavy sedation drugs used by veterinarians. The other birds will be awakened by the motion and noise of the first bird being killed, if they see and smell blood or another bird acting distressed, they are not just going to go back to sleep. (Despite the laziness and apathy of most meaties!) It is best if the birds being killed are not killed in front of the other birds that are still alive. Confine them to a small pen/coop/crate/box calmly before the event so you don't have to chase them to catch them. If they're in a large area, move them to a smaller one the night before, catch them while they're sleeping because it's easy. As you butcher them, take them one at a time, calmly, without making a big fuss, and kill them away from the others. I don't know that a mild sedative/muscle relaxant would cause a dying animal not to flap or struggle. Animals which are euthanized by an overdose of anaesthetic often flail around pretty disturbingly, even after they are unconscious, and they have muscle twitches after death. This is commonplace enough that veterinarians will warn owners who want to be present while their animal is euthanized that it might look like the animal is suffering because it can thrash and make strange noises, but the animal is already unconscious by that point.