Sharing screenshots of private conversations without the consent of the parties involved is not only wrong but may amount to certain civil and criminal liabilities.
To begin with, it is an invasion of someone else’s privacy which legally amounts to payment of damages. Then, there’s the Data Privacy Act which can mean jail time, considering it is a special penal law.
Now, if the screenshot turns out to be defamatory too, the original poster can potentially be held liable for cyber libel. Under the law, libel has four elements, namely, defamation, malice, publication and identity.
So if you post a screenshot, expose the identity of the individuals involved and provide a defamatory caption with the intention of subjecting them to ridicule and embarrassment, then you obviously commit the offense. Incidentally, even if there is no caption, the element is satisfied if the content of the screenshot in itself is defamatory and damaging in character.
By the way, I should stress that sharing posted screenshots or making mean comments apropos the import or the subject matter of the post does not make one liable for cyber libel. This is why you have to resist the temptation to post one because surely that would be shared gazillion times by the public which does not have to worry about the legal consequences. Certainly, the more your post is shared, the more you crucify yourself.
The lesson here really at the end of the day is you do not just post a screenshot without asking permission. If you feel aggrieved by what the subject individuals have done against you, the best remedy is still to go to court and not report the same to this seeming court of public opinion that is social media, where vultures and virtue signalers are salivating to destroy them. This may be morally just as some do gooders would put it, because you are exposing their incorrigible deeds. Legally though, this is not tenable because the totality of evidence, plus context, is what’s needed to paint a complete picture of what exactly transpired. This is something a mere screenshot would not be able to provide.
At any rate, should you have screenshots of certain convos, you can use them as evidence in court, subject to compliance with privacy, data privacy and electronic evidence rules. Their admissibility would have to depend on a number of factors such as authentication, integrity and relevance.
If you obtain the screenshots without consent like when the person has left his phone or is away from his laptop, or by illegal means such as hacking, or unauthorized access, then you won’t be able to use them as evidence because privacy is a fundamental right.
The only exception to this rule is when the person has given you password access to his Facebook, Instagram or Messenger. If that’s the case, he has lost a reasonable expectation of privacy over the contents of his account. So even if you do not ask for his consent, you can download or take screenshots of any of said content and present them as evidence. That’s the only way you can authenticate them electronically. Otherwise, they would be deemed hearsay and, therefore, inadmissible.
Be that as it may, should you encounter a screenshot that is crucial to your case, the best route still is to ask for a subpoena or judicial order. Unauthorized access to private communications is an offense under the Cybercrime Prevention Act. It makes no sense to take the law into your hands.