After doing some research on Safiyyah it seems like this thread is too quickly jumping to conclusions. From her actions in the muslim community after Mohammed's death it does not seem like she resented him for killing her husband. I also doubt she had a very good relationship with her husband as he hit her after having a dream which he interpreted as a desire for her to marry Mohammed.
And this tradition was recorded when? Exactly. Much later after said events, by whom? Surely not the husband as witness. Safiyyah, or anyone else for that matter, could've, and probably did make this up after the fact, to smooth things over nice like butter.