I don't think that anime and by extension AMVs have much connection or relevance to the massacre. This was a horrible, horrible thing that happened and even as an AMV creator... especially as an AMV creator... I feel that any attempt to memorialize the event using an AMV would at best seem poorly conceived and at worst, look like you were attempting to get extra votes/attention by connecting it to the shootings.
If I were to see an AMV by someone I didn't know, I would assume the latter was the case, and I wouldn't ever want anyone to assume that of my work.