Mostly it is to avoid contaminating nature with modified animals. Someone I once knew (working in Biology) had to kill 100s of them when covid hit and the labs shut. They are usually asphixiated by gas.
I recently saw a video encouraging people to adopt former lab mice. That made me think that the mice that are not adopted out might also be killed even if that is not needed for the study.
That’s specifically for the purpose of studying the body and brain as part of the experiment though.
Mostly it is to avoid contaminating nature with modified animals. Someone I once knew (working in Biology) had to kill 100s of them when covid hit and the labs shut. They are usually asphixiated by gas.
I recently saw a video encouraging people to adopt former lab mice. That made me think that the mice that are not adopted out might also be killed even if that is not needed for the study.
I would wager they were either surplus mice or part of control groups.