I want to write a story about a life that existed once, however, I'm afraid to get some people in trouble. What I want to say might help get somebody out of a bad situation and, at the same time, might direct negative attention on me which is what I don't want. Can I write about a nonfiction story without revealing actual names of the characters in it? Thanks.
You mean "based on a true story"? Let's assume the worst, and that the character whom you blacken decides to sue you for libel. Your defence would (from what you say) presumably be that it was all true. Could you prove it? If not, then I'd suggest that you need to change (at least) the names, and include a disclaimer to the effect that "this story and its characters are entirely fictional and not based on anybody living". Even if you have proof that would stand up in a court of law, do you have the resources to maintain the lawsuit as long as the offended party? Again, don't take the chance of a lawsuit unless you're prepared to go there. Bear in mind that a defence of justification (it's true, your honour) will increase any damages that are awarded because you are repeating the libel, and to a larger audience. It's possible - even if you do include a disclaimer that it's all fictional - that you will be sued but, here, the onus would be on the plaintiff to prove that it really was him that you meant, and he then runs the risk that - especially if he can't prove it - the libel (even if untrue) will be believed because "nobody knew it was him until he stood up and shouted 'it's me!'" Bottom line, make your fiction fictional - even to the extent of writing it as, say, sci-fi - unless you hanker after your day in court.
My advice, consult a literary lawyer. An internet forum, well-meant though we are here, cannot seriously give you legal advice. Anything we say here, you would need to verify, so save all of us the trouble and just go straight to the lawyer
In this regard I always wonder how did Goethe view his book basically killing many young men who, same as Werther, comitted suicide when following his example. Not that I'm presenting you with advice, I just understand your concerns.