There was this guy - he was a religious fanatic; that means that he does bad things - even kills people - in the name of God. And on September 11th of 2001, he told a bunch of his followers to pilot airplanes into buildings in New York. They did and about 3000 people were killed, on the planes and in the buildings. And so we - our government - sent some soldiers to the country where he was hiding, and now, ten years later, our soldiers found his hiding place and killed him and brought his body back so they have proof. This is not a nice thing for a country to do, but this guy was really REALLY bad, so it had to be done.She seems to get it. But then her favorite fiction helps her to understand that good and nice are not synonymous. In Teen Titans, many of the episodes are about the hard choices - is what's right or good the same thing as the nice thing to do? - especially for Robin and Raven - and she's a Raven fan. The character of Raven herself is explained by my favorite website as "Chaotic Evil by nature and Lawful Good by nurture" - that is, her father is a demon and she is therefore evil by birth, but she's a superhero as part of a superhero group, and while certainly dark, she is not evil. I think that's what Abby has in mind when she hears that somebody - even a country - did something that was not nice but that must be done for the good of the world or the galaxy or whatever.
And in Harry Potter - mind you, she's only seen the first two movies, and we're halfway through the first book - the only not-nice actions by good characters she's seen are by the Weasley twins, and those are more in the nature of pranks than evil. She has not yet seen the things that less cutesy characters can do - she does not understand Dumbledore's outright manipulation of Harry, for example (not-nice in the name of greater good), much less the more complex behaviors of double-agent Snape. She's only eight. She does not need to be there yet. In the meantime for these difficult talks, well... there are always the DC Universe's younger denizens to use as our analogies.