There would be some instances of violent conflict in a world with only very good people. Due to differing information, two good people could have opposing duties in such a way that they'd have to fight and neither of them would be in the wrong. I call this situation moral conflict. (Some freeloading situations are a non-violent example of moral conflict.) Protagonists would understand this and take it no further than the subject of the dispute was worth nor harbor any hard feelings over it, but I can't say the same for non-Protagonists.
To give a non-real-world-politics-infected example of a situation where the moral conflict would be taken to the point of violence, consider two good people trapped in a cave and knowing when they'll be able to escape but having only enough food for one of them to survive that long. The moral course of action (for either agent) is to try to save whichever of them they think is more important. In most cases, this will be themselves, as they're very likely to have at least some disagreement in how a person should act, but obviously both people think they have the right side of every disagreement. So if they both understood the lesson of this article, they'd fight to the death in complete good spirits and without any anger between them. (We're discounting the risk of this resulting in both of them dying.)
Abortion and veganism are the most common contexts this is ignored in real life. Tons of people say things like "If you think it's immoral, you don't have to do it, but you do you and I'll do me". That's right; if you think I'm killing an innocent creature that has rights, just don't participate and stop complaining! Nevermind the fact that *they are forcing you to participate* by funding it with tax money...
The historian's fallacy is a form of this.