I've never seen or hear anyone I know use Keybase.io for actual texting, and I work at a large software company. Everyone just uses it for PGP keys, then forgets it exists.
It's not homicide nor manslaughter neither, and has no concept of "premeditation" at all. It's was a 10 page years old thread on an obscure gossip website.
He was not doxxed (no one even knew his name), did not have threats posted there... I fail to see how it can even closely be related to "murder".