Children are naturally fairly black and white in their views of right and wrong.
I've been discussing this - with adults and children, in person and online - for the past fourteen hours, as regards the assassination of Osama bin Laden. And although it may have been "justifiable homicide", it was assassination nonetheless (in my opinion; I have only news reports to go by - it may be that the soldiers gave multiple chances to surrender peaceably and he chose the martyr route, in which case, suicide by cop). I tried to explain it here to my eight-year-old, and she seemed to grok. But then this morning she still thought we should "celebrate, because he was a bad guy". It's true. He was (at least from my point of view and that of most of my country). But killing people - no matter what your motives - is at the very least a grey moral choice. It may be too subtle a distinction for Abby to grasp right now, at eight; Bad Guys get captured or killed and that is The Way The World Works.
I tried to explain it in terms she understands - her favorite characters don't go around killing people and then gloating about it, for instance. Mostly they try not to kill at all, except for mercy killings at the dying person's request, or by killing things that are not even people, much less human. So it's not usually a good choice to kill people, no matter how bad they are, but our government felt it was necessary - and has felt this way for longer than she herself had been alive.
If she has not digested this by the end of the school day, we'll have to break out the big guns - my favorite Sondheim musical, for instance, where destroying the giant in the first act wreaks havoc upon the whole area in the second... because the giant's loved one retaliates. I don't want to frighten her into believing that bin Laden's people will, say, bomb her elementary school. It's highly unlikely that we are important enough in the minds of people with that kill-them-all-and-let-God-sort-them-out paradigm to be a tempting target.
But she should be aware - we should all be aware - that killing someone - even if it's justified - is gonna piss somebody off.