If you wish to deplatform based on broadcasting a killer's message then a lot (probably almost all) of newspapers would have to be closed.

Consider Zodiac killer's letters or Breivik's manifesto. If copycat killers arise saying they were inspired by reading their letters/manifestos in San Francisco Chronicle/BBC respectively, should these media outlets be closed and/or deplatformed?

This seems to be no different than wanting to ban/close car manufacturers for 2016 Nice truck attack in France, leaving 87 dead.