Nick Sandmann's family is suing the Washington Post for libel. When examining the Washington Post's reporting about Nick Sandmann's altercation with Nathan Philips and others at the pro-life rally, and examining it from what common law has to say about it, and not from an emotionally biased and political position, it becomes clear that the altercation is textbook libel, and possibly even a criminal matter. This was libel, and possibly criminal libel, because people all over the internet started calling for violence against this child and his family, as well as actions against the school, as a result of their shoddy reporting.
The Washington Post was not reporting the "facts", since they didn't perform due diligence to ensure that the so-called "facts" were even correct. A video that surfaced a day or two later showed that it was a completely different story, with Nathan Philips provoking the altercation, and subsequently lying about it to the press. The Washington Post failed to act responsibly by failing to perform due diligence and discover the actual truth, which is what investigative journalism is all about, and they therefore needs to be held accountable for their poor conduct and lack of institutional control over their reporters.
To understand why it is libel, and possibly even criminal libel, see this excerpt from Black's Law Dictionary, 2nd Edition 1910, posted below. This is not a free speech issue, as libel, criminal libel, and the first amendment of the US Constitution all seemed to coexist for hundreds of years without it being a contradiction. Free speech and the freedom of the press does not allow one the freedom to ruin another person's life without an underlying truth based cause. While Nick Sandmann is free to ruin his own life based on his own behaviors, and that can be reported on, somebody else cannot fabricate a story and then seek to ruin his life. This kind of behavior is considered aggressive, and therefore unlawful and a violation of Nick Sandmann's natural rights.