A conviction on manslaughter is an outlier
Under Massachusetts law, a person has been convicted of involuntary manslaughter for encouraging another person to commit suicide with words alone. This conviction was upheld on appeal to the Supreme Judicial Court of Massachusetts. The Supreme Court of the United States declined to hear the case. See Commonwealth v. Michele Carter, 481 Mass. 352 (Mass. 2019); petition for certiorari filed July 8, 2019; certiorari denied January 13, 2020). I can find no other example, and counsel for Ms. Carter said to the Supreme Court of the United States in 2019 that there was no other example.
The petition for certiorari said:
Massachusetts is the only state to
have affirmed the conviction of a physically absent
defendant who encouraged another person to commit
suicide with words alone. Before this case, no state had
interpreted its common law or enacted an assisted-suicide statute to criminalize such “pure speech,” and
no other defendant had been convicted for encouraging another person to take his own life where the
defendant neither provided the actual means of death
nor physically participated in the suicide.
You correctly identified two issues: (1) causation; and (2) a possible First Amendment violation.
The facts are disturbing, so do not read on if you do not want to read about a person's death, but it is important to recount the facts to understand how far above mere bullying this was. The Supreme Judicial Court of Massachusetts wrote:
Although we recognize that legal causation in the context of suicide is an incredibly complex inquiry, we conclude that there was sufficient evidence to support a finding of proof of such causation beyond a reasonable doubt in the instant case. The judge could have properly found, based on this evidence, that the vulnerable, confused, mentally ill, eighteen year old victim had managed to save himself once again in the midst of his latest suicide attempt, removing himself from the truck as it filled with carbon monoxide. But then in this weakened state he was badgered back into the gas-infused truck by the defendant, his girlfriend and closest, if not only, confidant in this suicidal planning, the person who had been constantly pressuring him to complete their often discussed plan, fulfill his promise to her, and finally commit suicide. And then after she convinced him to get back into the carbon monoxide filled truck, she did absolutely nothing to help him: she did not call for help or tell him to get out of the truck as she listened to him choke and die.
The Court rejected the argument that this conviction was a First Amendment violation (citations removed):
We disagree and thus reaffirm our conclusion in Carter I that no constitutional violation results from convicting a defendant of involuntary manslaughter for reckless and wanton, pressuring text messages and phone calls, preying upon well-known weaknesses, fears, anxieties and promises, that finally overcame the willpower to live of a mentally ill, vulnerable, young person, thereby coercing him to commit suicide. We more fully explain our reasoning here. The crime of involuntary manslaughter proscribes reckless or wanton conduct causing the death of another. The statute makes no reference to restricting or regulating speech, let alone speech of a particular content or viewpoint: the crime is "directed at a course of conduct, rather than speech, and the conduct it proscribes is not necessarily associated with speech." The defendant cannot escape liability just because she happened to use "words to carry out [her] illegal [act]."
Lesser charges are available
That being said, for all of your questions, there are also lesser charges that might apply, such as harassment or inciting suicide (e.g. California Penal Code § 401; or for example from another country, see Canada's Criminal Code, s. 241).
The other two answers likewise describe offences other than manslaughter or murder, in Australia and England and Wales.
