I have read the book. The point of that anecdote is that the concept of "cruelty" does not apply to the authors' thinking. We say she was being cruel to the animal, but she does not see it as such.
To her, she had a problem: something was in her swimming pool, and it had to be removed from the pool so she could keep it clean. She tried to remove it, but it kept swimming away. So she drowned it, collected it, and cleaned the pool. The creature's pain never entered into the calculation for her.
The best analogy I can think of are open-world videogames. When you play Grand Theft Auto, are you being "cruel" to civilians? They aren't alive and don't have any feelings, so you may have no problem [1] doing seemingly cruel things to them to see how they react. You're not necessarily cruel, you're just curious how that world works.
Sociopaths see the world in a similar way that you see Grand Theft Auto.
[1] I'm empathetic enough that I feel empathy even for completely non-sentient characters in videogames. So I tend to be a goody-goody even in digital worlds.
I think it's a side effect due to the lack of empathy. On the other hand, they do enjoy their domination over weaker beings, so this might be pleasurable to them.
