How else might the irony of Sylder and John Wesley's friendship be interpreted?

One might interpret this in a few ways. First, it might be meant to show the senselessness of revenge. If John Wesley discovered that Sylder killed his father, it would not change who Sylder was, even if it changed how John Wesley perceived him. Sylder would still be the friendly and somewhat sympathetic character that he is, hardly deserving of John Wesley's anger. To put the same point in another way, if John Wesley knew that Sylder killed Kenneth Rattner, it would not make him see Sylder more clearly, but instead, less clearly. Then he would be blinded by rage and his quasi-demonic oath of vengeance.