[srean]

Why not experiments on convicted criminals, inmates, prisoners of war, political prisoners ...

The line is somewhat arbitrary and personal. It makes sense to the person.

To be honest, I don't know what decision I will take if a trolley problem is jumped on me without prior notice.

[erikerikson]

As you write, we all have our own balances. I'd suggest there are ethical reasons against those.

As a society we have investigation review boards, laws, and other conventions.

There is a sizable set of people that like to outsource (want it done but not to do it) which seems dishonest, like the town butcher in Buddhism bring a bad karma sink.

[srean]

Indeed.

In the particular non-hypothetical case I mentioned, the lab monkeys were a little too close to human babies in their behavior and appearance. That did not help at all.

Those who do not want partake in specific class of animal testing should also recuse themselves off the fruits of that class of experiments.

[erikerikson]

That lab sounds terrible.

In the only monkey lab I've been in, I witnessed deep mutual love between the animals and their keeper. The conditions you describe seem unnecessary.