What happened to the monkeys is secondary to Elon not understanding why and what would happen to them: He publicly stated the monkeys were terminally ill. They were not. That's not how anything works. If you don't grok that, you should be nowhere near decision making about animal testing.
Evidently Elon did not understand that "terminal" meant the test animals would likely or certainly be euthanized if they did not perish due to the experiments being run: https://futurism.com/neoscope/elon-musk-neuralink-terminally...
That said, there are standards for animal suffering in research, and that should be checked as well.