> Can we justify experimenting on a known serial killer if the experiment will save a million children's lives in the next 10 years?

I mean, for one thing you can’t breed transgenic killers with a desired genotype by the millions.

> If an alien species were using us for their medical experiments, under the same circumstances and conditions, then what would I say?

That’s too hypothetical. Aliens will be more similar to their native fauna genetically then to us.