https://en.wikipedia.org/wiki/Imminent_lawless_action

https://en.wikipedia.org/wiki/Brandenburg_v._Ohio

I think there's a relatively strong argument that this girl's speech was intended to incite imminent lawless action, but that only means it's not protected by the First Amendment. So maybe we don't have protected speech here, but what crime have we committed?

Massachusetts statutory law does not define voluntary manslaughter. Rather, Massachusetts common law, as announced by the courts, provides a definition for manslaughter:
The unlawful killing of another, intentionally caused from a sudden transport of passion or heat of blood:
(1) upon a reasonable provocation and without malice or upon sudden combat; or
(2) from the excessive use of force in self-defense.

As with voluntary manslaughter Massachusetts statutory law does not define involuntary manslaughter. Rather, Massachusetts common law, as pronounced by the courts, provides the definition for involuntary manslaughter:
One can commit involuntary manslaughter through:
(1) an unintentional killing occasioned by an act which constitutes such a disregard of the probable harmful consequences to another as to be wanton or reckless; or
(2) an unintentional killing resulting from a battery.

Based on a quick google of what constitutes manslaughter in Mass, I don't think we have manslaughter. At best you can maybe argue the first point of involuntary manslaughter, but then you have to make text communication not just unprotected speech, but an act, AND an act that was in disregard of PROBABLE harmful consequences. I don't see it.