I always see these articles about people leaving malicious comments. But I want to know what they actually said. How bad can a comment be that you get legally punished?

The freedom of speech is a sadly a very misunderstood right (thanks largely to a wretched public education system). In the USA, the freedom of speech means that you cannot be arrested or sent to prison by the government for what you say. In contrast, in many totalitarian states without such a right, if you criticize the government or its officials, they may arrest and imprison you.

Contrary to popular belief, the freedom of speech does not give you the right to say whatever you like without consequence. If you knowingly spread false or defamatory rumors, you may be held accountable for any damages you cause to individuals or organizations. The damaged parties may sue you in civil court for liable or for defamation of character. Your private sector employer is fully within their rights to fire you. This is not a violation of your civil rights; you have no right to harm others with falsehoods.