Writers are not necessarily friendly with other writers all the time. What with the competition for sales and prizes, some good-natured professional rivalry is to be expected.
If you write fiction for an extended period of time (up to a decade or more), it is almost certain that at some point in the future, some other writer will pick a fight with you. Not a REAL fistfight, that is, but with words.
So writers get into feuds with each other. The examples are many, all of them embarrassing. Why squabble when the stakes are so small? Why do great minds succumb to petty impulses and get involved in long, bitter feuds -- often over tablescraps?
When things go really wrong, writers sue each other. The authors of HOLY BLOOD, HOLY GRAIL sued Dan Brown for allegedly stealing ideas from them. The poor fools lost the case and a great deal of the profits they had earned as a direct consequence of THE DA VINCI CODE helping sales of their book.
Writer feuds are mainly about pride, not money -- and thus they extend beyond the grave! When a writer dies, especially a successful one, jealous rivals will come out and badmouth his/her memory. When a long-dead writer receives some public honor, a jealous colleague will dish out some dirt on the dead rival, in the vain hope that it tarnishes the glory.
There is no law stating that you must treat all other writers' work with reverence, no matter what -- after all, if you make your writings public, you do give others license to criticize.
But avoid feuds. They get too personal, and suck creative energy out of your life that should have been used for better things. If another writer tries to pick a fight with you, simply ignore him. Don't get mad! It's not worth it.
If you have resentments and grudges, you can always channel them into your fiction...