I don't know the origin or anything like that. I'd suggest just calling the tag murder is okay for now, unless it has an official name. Whatever it is though, create a stub wiki and add to it as more information gets found out, to include the tag terms that other sites like Pixiv or Twitter uses.
Seems like that may be what it is called. One of the replies on one of the posts from above (twitter #1439963612626374664) pointed out the original image, which is from the series Hamtaro. The origin for that image comes from twitter #1059198057089101824, which is based off of a reddit post (https://www.reddit.com/r/wholesomememes/comments/8aekd4/important_psa/).