Ethic Dilemma

"If they live together, they die together. This is just a way to reduce the unnecessary stress they experience before they die."The ARP people told me.
"So I just kill them all."
"No, you're not killing them, you are SACRIFICING them."
"Yes, "I convinced myself,"for the welfare of human being, we have to scarify them."
Carbon dioxide began to accumulate in the transparent box, the mice started to choke. Couldn't endure to see this, I turned around.
"Did they die in torment?"
"No. Some researchers may have a morally guilty feeling on seeing their suffocation, but actually they already lost consciousness when they suffocated, thus there was no suffering for them."
But I still couldn't get rid of the sense of uneasiness: Can this be true? How can we know a mouse's feeling before its death? Suddenly I thought of what happened during World War II in the concentration camps.
Finally all mice stopped moving.
"Cervical dislocation."
"Haven't them died?"
"Yes, but in case they revive once there's enough oxygen. You definitely don't want to see them wake in cold and darkness, sustaining themselves by eating the bodies of their own kind and finally die in fear and desperation. This kind of thing once happened."
After cervical dislocation, all the bodies were sent to cold room. Even when this kind of thing has become part of daily routine, I still remember every detail of my first lab animal training. Even everything we do all exactly according to the SOP, to put an end to life is definitely not an enjoyable thing.
I used to have the impulse to take one mouse out of the destiny of death, and keep it myself-they look so lovely and smart-but I can't. They don't belong to the normal, wild world. They never exposed to the open air since the day they were born, not to mention see the beam of sun. The only reason they were created is for scientific research, since they are highly inbred strains and usually carry some specific gene mutations, which make them ideal model for some human diseases. Once leave the barrier system, they couldn't survive.
So I can just pray, pray for those innocent creatures to rise to Heaven after their death, but someone once told me that animals have no soul. What a pity! How dull it would be if there are only humankind living in the garden of G_d, without any animal?
This is an unsolvable ethic dilemma on the issue of lab animals.