You want to have another look at Chapter 19 of this great book, which is where Kit is tried for supposedly being a witch. What is interesting is that William, presumably concerned for his public name, does all he can to distance himself from Kit in her time of need and does nothing to try to support her. Nat, however, even though he has been told he will be punished by being flogged if he returns to his village after his stunt at Halloween, comes back and risks himself to bring Prudence to the court and to reveal the truth, saving Kit from an uncertain future and certain punishment. Note what Goodwife Cruff says when she transfers her anger and rage from Kit to Nat:
Isn't he the seaman? The one who was banished for setting fire to houses? Thirty lashes they promised him if he showed his face here again!
Nat, by this act, clearly shows his love for Kit. William shows himself to be the coward that he actually is, who is so obsessed with public standing and what others think about him that he ignores Kit. Obviously, for William, society's view is more important than that of his own heart, which is definitely not the case for Nat.