One reason Better Call Saul is such a good spinoff is a smart rule invented by star Bob Odenkirk. It's not always the case that actors get to weigh in much on how their characters should be portrayed, at least at first. Show creators, writers, and producers generally have an idea of how they want a character to behave, and how they want that character to be perceived, long before anyone actually steps in front of a camera to shoot the first episode.
In Better Call Saul's case, though, it makes sense that Odenkirk's opinions and ideas would be given their due weight by those crafting the show, as Odenkirk had made the Saul Goodman character his own over multiple seasons of Walter White and Jesse Pinkman needed a criminal lawyer to get them out of a jam in season 2, and quickly became a fan-favorite member of the extended Breaking Bad cast.
The thing was, Saul was a pretty scummy person as depicted on Breaking Bad. While he was funny and entertaining, he often behaved in immoral - or at least amoral - ways, willing to do all kinds of awful things for his equally morally compromised clients. That's likely why Bob Odenkirk came into Better Call Saul with a single demand for co-creators Vince Gilligan and Peter Gould: Saul, aka Jimmy McGill, had to be a likable character. Odenkirk didn't believe Breaking Bad's Saul was a likable person, and if the spinoff was going to be a success, he felt Jimmy needed to be presented in a more likable light. This rule has definitely paid off.
While Breaking Bad itself proved through the moral devolution of Walter White that a show doesn't necessarily need a likable protagonist to be great, Better Call Saul is a different beast. Walt at least began Breaking Bad as a man viewers could empathize with and root for, before finally crossing the moral event horizon. If Better Call Saul had started out as the story of a man beyond redemption, with no moral fiber left, the spinoff wouldn't be even close to as memorable and acclaimed as it's become. Even in Better Call Saul season 5, when the Saul Goodman persona has moved firmly to the forefront, Jimmy McGill is still clearly there deep down, leading Saul to struggle with some of the increasingly bad things he's being asked to do.
Jimmy is also getting worried about how his influence seems to be twisting Kim Wexler further and further down the path of moral turpitude herself, to the point of even being shocked by her suggestion that they outright ruin Howard Hamlin's life and career for fun. These moments remind the viewer that Saul still isn't without empathy and comion, and allows audiences to stay invested in Saul/Jimmy as a character and person, rooting for him to both succeed and find redemption. That's despite knowing full well what a sleaze he ends up as during Breaking Bad, and the purgatory of a life he enters after, as Gene in Omaha. Everyone watching Better Call Saul knows where Saul's life is headed, but even with that knowledge, they hope he'll somehow make the right choices this time. That wouldn't be possible without adhering to Bob Odenkirk's golden rule.