I haven't seen every "Rifleman" episode, but this one would have to be in the running for "absolute worst". Indeed, it is surely among the worst episodes of any Western TV series ever made.
TV Westerns often had episodes in which bad guys show up and cause all sorts of trouble, which is (usually) resolved by gunning them down. "Gunsmoke" was a generally thoughtful and well-written show, and when it did this sort of thing, it did it well. "The Rifleman" too-often did it poorly.
This episode is an atrocity. Three brothers arrive in North Fork, get drunk, and tear up the saloon. When Micah and Lucas arrive to stop them, one of them fires his gun at Lucas, but accidentally kills his brother. (Naturally, he blames Lucas for the death.) Thrown into jail, they warn Micah and Lucas they'll be sorry when their father shows up.
Which he does, and they are. The father, an utterly unrecognizable Buddy Hackett, at first appears to be disturbed by his sons' behavior and tries to make things right. But, as they, he's no more than a "psycho on a spree". The story ends with the survivors thrown in jail. And that's it -- nothing else.
This is the cheapest, sleaziest, most-pandering sort of writing. There is no point whatever to the story, except violence, violence, violence. Contrast this with a similar "Gunsmoke" episode, "Matt Dillon Must Die!", in which a crazed old man and his sons seek revenge for the death of his belovéd wife. Yes, it's dark, violent, and nasty, but the characters have some degree of complexity, and we're interested in why they act the way they do and what happens to them.
"Bloodlines" stands in stark contrast to the immediately preceding episode (2.1, "The Patsy"), in which the town barber (Whit Bissell) is forced to decide between his own life and his son's, and Lucas has to decide whether he's willing to look cowardly to prevent a killing or two. This is "The Rifleman" at its best, when its characters have to make tough moral and ethical decisions.
I would normally give this episode 1 star (and even that is generous), but it gets 4 stars for Hackett's outstanding performance.