The whole point of the ending was Jimmy to stop running away from his demons, lies, facades, mistakes, consequences, victims etc.
Him and Walter are narcissistic in their own ways and their egos and thus ambitions were their greatest fuel but also their greatest fall.
Chucks death should have been what made him stop but he was already "doomed by canon" as the trope goes, due to this being a prequel.
IInstead he truly embrace the dark side, his Saul personality and doubles down as a coping mechanism.
Chuck was the one who constantly belittled and criticised him and Jimmy did SO much to care for him, let's not even go into the cluster fuck that was "Chuck's condition" but Chuck Is like that overbearing parent, the too judgemental/critical parent who is never satisfied and demanding more from you.
Chuck also had extremely strong virtues/beliefs and he clung onto those with just as much ferocity.
When he died, he "freed" Jimmy in some ways and in his grief over his brothers death and how and why too, the true realisation of who, what and why drove Chuck to such not just suicide but such a brutal suicide method too, it wounded Jimmy in a way that he could never heal or recover from, that he embraced those immoral lawyer and salesman personas and reinvented himself.
All the negative feelings Chuck and others made him feel, he weaponised it against the world and he overwhelmingly tried to compensate for never feeling good enough, just like Walter did.
Insecurity is usually the driving/defining force of narcissism amd thus the human condition.
By the end of BCS, Jimmy fucked up too much. His actions in BCS and BB ruined and cost far too many lives including innocent ones. Seeing Howard get killed mercilessly, unfairly, instantly, coldly, unjustifiably etc like that traumatised him too.
Like the death of Chuck this was another breaking point and test of character for him and he had to stop, he had to stop running.
He was ruining Kim's life and she ruined his too. They were both toxic, selfish, reckless, self absorbed, immoral etc people but far worse together and had strong chemistry for a reason too.
Both Walter and Saul realised they were the problem, their overwhelming egos were the true poison and they had to stop or be stopped regardless once and for all.
Walter was far worse than Saul, becoming another Gus and not so different from the cartel either. So he got a much worser fate but he was also dying and had nothing to lose anymore, as he had everything he needed and wanted but ego, insecurity and thus greed ruined all of that and everyone around him like nuclear radiation and explosive chain reactions too.
Saul didn't kill people at all IIRC but he was still directly involved, still helping, supporting, protecting these kind of scummy, inhumane criminals and he had a LOT to answer for too.
He may not have killed anyone directly but he caused a fair few deaths indirectly/unintentionally at least and ruined many lives too.
Saul alway ran from consequences, punishment and the uncomfortable truths. He needed to face them for once and grow up. To realise he fucked up and atone for them.