Tests with animals have shown that concious life is gone after 2-3 seconds because your brain stem is cut and you stop getting blood. Showing signs of life doesn't imply that there is still a person in there.
Animals aren't human and it's very hard to know EXACTLY when conscious life stops. Assuming that's the best data we have it's still an incredibly barbaric practice.
Not really, we don't expect judges to be prison guards either.
Yes because that's totally the same as ending another humans life.