I didn’t mention necessary evils in my main comment. With necessary evils, the pleasure is more of a secondary effect to relief of some kind. The fact we even have the phrase in our language for the concept and that it contains the word “evil” would seem to serve my point.

Also, consider that the harm is minimised by the fact that those bacteria cannot suffer in any meaningful way.

For where the harm is greater, tens of thousands of pages of law have been written about where the line between pleasure and relief falls. How evil an act is deemed to be is ultimately indicated by the severity of the enforced consequences (prison sentence, etc.). Or at least that’s the idea.

Maybe it was a man-on-man killing by someone in danger with no other apparent reasonable options. Maybe they just felt like it and are only saying that. Maybe they were clumsy or incompetent. Was that with malice? etc. etc.

And then there’s those enforced consequences. They’re also a necessary evil.

But these are all evils.