Luke Cage was a great show… almost all the way through. Cottonmouth was a great villain, in that he felt real. He was even sympathetic. You understood why he was going down the path he was going down, you got that he wasn’t an intrinsically evil person, but that he had a past he had to overcome. Warning, spoilers ahead!
Then they killed him off and replaced him with Luke’s half-brother/former best friend. They made Diamondback (the half-brother) be the author of all Luke’s problems, and had him just pop back into existence. It made the story small, smaller than it should have been. Despite the neighbourhood focus of the show it always felt like it was part of a larger world until that moment.
This is a common thing in fiction, and it rarely makes the story better. It’s a lazy attempt to raise the stakes, to bring personal involvement into things… but make no mistake, it is lazy as all hell. The last time it was done well that I know of was the line “No, I am your father”. Even that is a bit suspect and mostly worked because Vader was the number two villain, not the number one villain.
It’s something comic books and comic book derived movies and tv are more guilty of than most other mediums. Marvel’s Defenders series relies so far on it for almost everything. The one time it worked was because it was the story. Kilgrave was established at the beginning as a figure from Jessica’s past, and what broke her in the first place. That meant it wasn’t a big reveal, the whole story was the intersection of these two people. The Punisher in Daredevil season two was possibly the worst of the bunch, although because it was a secondary story it wasn’t quite as damaging to the narrative as Diamondback. Iron Fist… well, let’s just ignore that. If you take away the “Surprise! Your close connection is the author of all the evil you are surrounded by” there isn’t a show anymore.
It feels more real if we go one of two ways, if we have the villain meeting the hero and establish their relationship on screen, or if we remove that history entirely. Look, Deadpool did a great job with this. Francis, the main villain, is responsible for Deadpool getting his powers, but all of that is on screen. There is no surprise history, no shocking reveal. Francis is just a dickbag, start to finish. That establishes a personal connection between them, but it’s one with weight because we saw it develop, we got to experience it.