Well, if he knew she was thinking of remarrying, I can see him sending Katherine Parr the way of certain other of his wives. Fidelity to him ( if you were his wife) was an all important virtue, and that's the reason Catherine Howard breathed her last. Katherine Parr would not have had to be physically unfaithful to him then to warrant a bad reaction, just knowing she intended to remarry might well have been enough. But he didn't know that, and had no proof. He would not have exceuted her for being a bit if an argument or annoyance when it came to talking religion.