Facebook user Peter Thoeny  brings this to our attention.
The targeted ad appears next to a status update by Thoeny, who says "I am mad as hell about this." He took to Facebook to write that the issue behind the shooting was solvable, but not by a wholesale ban on guns. He went on to outline some specific, level-headed ideas. Then he saw the ad. And he posted a comment: "This is a sickening and offensive targeted ad I just got on Facebook! These violent computer games are IMHO one of the MAIN REASONS we have school shootings like this. Thanks for listening to my rant."
Below, an image of Thoeny's annotated version of the ad.
We've written to ask Facebook how it handles these situations and whether there is any institutional procedure for dealing with this kind of ad adjacency. When we hear back, we'll update this post.
UPDATE: Facebook tells us: "The ad in this screenshot is targeted to the user based on demographics and interests. In this case, it’s a male-oriented video game so the targeting criteria could have been as broad as Males 18+. Targeting is not related to the content of your newsfeed and this ad is not shown in connection with the newsfeed story at all."