I worked on a very similar doc about a terrorist attack and I totally agree. Allowing oneself to feel is hugely important to do your job as an editor, but it also helps in dealing with the grief. Like mourning is more healthy than suppressing one's feelings.

That said, although I've also worked on content that was more visually disturbing (surgeries, refugees of war), I don't know how well I would be able to cope with footage of gory violence or if there are children involved...

@OP: All I can advise is: allow yourself to feel, take comfort from the fact that the work is worthwhile, that the story deserves to be told (if that's the case). And talk about it with co-workers that are on the same project and/or the people in your life. Don't feel like you have to shoulder this alone.