Doesn't the context of billionaire endangered animal hunter inform us that it may possible be not-serious.

Surely?

On the other hand, there's bound to be people who believe fair-trial-judicially-decided capital punishment would be fair retribution for anyone who intentionally kills endangered animals.

I'm going to say something like: I'm against capital punishment because it tends to kill innocent people at least occasionally, not because it isn't often deserved.