It is possible to write about a person—even in a negative light—without having to ask their permission. However, if what you say is presented as fact, you must be able to verify it as the truth. On the other hand, if it is offered only as an opinion, that is protected by the First Amendment. Making a statement as fact—that you can’t prove—may open you up to a libel suit.
Realize that you can write negatively about a person using a different name, but if they are readily recognizable from the incident or situation described, you can still be open to libel charges.
It is always best to use two criteria for avoiding libel suits. From a legal standpoint, never write anything for publication that is not true and verifiable. From an ethical standpoint, always ask yourself if there is anything to be lost or gained by using their real name.