Massa and Missis Murray are the family's final owners. They're city folk who only own their plantation because of an inheritance, which makes them the most sympathetic slave-owners in the novel.
Of course, the "most sympathetic slave-owner" is like "the least slimy slug"—we're dealing with subtleties so small you need a microscope to see them.
Still, those positive qualities are worth pointing out. The Murrays never hire a real overseer for the farm, for example, which is great because overseers are basically paid to be violent towards slaves. We can also see it in Massa Murray's reaction to Tom getting beaten while working for the army: he immediately believes his story and promises that he'll never have to go back.
That's cool and all, but nothing can change the fact that they own other human beings. That's what you call a deal-breaker. What's more, the Murrays seem totally oblivious to this fact, as evidenced by Massa Murray telling Tom that he "looks forward to [them] enjoying the rest of [their] lives together" after defeating those "human devils" from the North (111.48).
Think about that for a second. It never crosses Massa Murray's mind that Tom might not "enjoy" being owned by someone else, nor that he might think the only "human devils" around here are those soulless scumbags who own slaves. This is a powerful and incisive critique of the rationalizations made by so-called "civilized" people who embraced chattel slavery.