The fourth pillar of Wikipedia says: "Editors should treat each other with respect and civility".
Considering that Wikipedia's contributors hold many different viewpoints, Wikipedia's success depends on its editors getting along peacefully. When they disagree, they are expected to do so reasonably.
In large part this is because an encyclopedia has only one article about each topic, no matter how controversial the subject may be. If it is to have any chance of being neutral, its contributors must work to find consensus, and that's probably not going to happen if everyone is constantly sniping at each other.
Wikipedia has developed many rules to explain what "respect and civility" means in practice, but it's tricky. Similar to the pillar about neutrality, this principle is not always strictly observed.