I really love the common interpetration of "John wipes a tear if he kills Javier" because it gives John so much more dept that's denied when he's forced to be this badass that never shows weakness and control his emotions, specially because he cries over someone he thinks don't deserve to it, a person that hurt and betray him, maybe there is regret? maybe there is nostalgia? maybe was the lack of real closure.

Thing is, it makes him human in a way "badass men never cry" could ever. It makes his broken friendship with Javier much more tragic. And while personally I do not think it's canon (if his actor was told to don't cry when Arthur dies because "John Marston does not cry". I assume is less likely for him do so when he was even more a macho stereotype). So I support it as a "death of the author" kind of take that actually enrich John as a character. And so does Javier, why not? How much have you done for John that even when he hates you, the part he once cared for you overpowers him for a little.

And it's so sad how many people would rather have John be this idealized man that's never dealing with ugly emotions. Or that think the headcanon makes no sense because he spits on Javier right after and somehow people is dumb and didn't see it. Like no, people saw it, is just that crying over a person you hate because they still once meant something to you, can cause those feelings. Is how many people feel after a break up or leaving a toxic relationship where they still had happy moments and loved each other.

The idea John not being able fully stop feeling some type of sadness and trying to hide it with a sign of disrespect is not unrealistic. Again, it makes him a human with complicated human reactions. And that's why people hold so much to that headcanon, or even see it as canon. Because it adds so much to him.