How is killing a living creature purely for your pleasure not diametrically opposed to cruelty?

The obvious argument is that it's not purely for pleasure, but again, I'm not even trying to argue that. You seem to be of the mindset that nobody is allowed to be against any animal cruelty if they aren't also against everything you personally consider to be animal cruelty. Save that conversation for another time.

Because the topic of conversation was the justifiability of the suffering of animals for scientific progress.

No, you insisted on the topic being only about animals. The topic started on experimental cruelty to animals, then shifted to the invaluable knowledge gained by that cruelty, then shifted again to a much more extreme case of invaluable knowledge learned from experimental cruelty. Conversations don't have to stay strictly within the parameters they start out with, especially on the internet.

That's disrespectful to the actual people who lived through that trauma.

Is it? Have you actually talked to anyone who lived through that trauma to see how they feel about animal experimentation? Or are you just being offended on behalf of people you've never met?

If the equivalence is so obviously different

Look, I've practically spelled out the similarities and differences already, but apparently I need to do it yet again. Nobody is saying that the humans=animals, or that the experiments were equal in cruelty, or that the suffering was equal. That should go without saying. They're similar because of overlooked suffering for the advancement of scientific knowledge.

there is no argument to be made by comparing them.

No shit. It's not an argument, it's a comparison.