When it comes to NCIS, there are a lot of reasons why Gibbs was let go. According to reports, the writers felt that they needed to shake up the show and decided to kill off one of their main characters. While this may be true, it's also possible that the actor who played Gibbs, Mark Harmon, wanted to leave the show. There have been rumors that he was planning on leaving NCIS for a while now. Whatever the reason may be, fans were shocked when they found out that Gibbs had been killed off.