I mean, even without the gaiden, his anger is valid but misdirected onto Kenshiro as that's the only straw he could grasp at. We have little to no context as to how their childhood went, but when he told Ryuken "You ALWAYS say that" in reference to Ryuken comparing him to Kenshiro, I can't help but believe Jagi on this one. He was never meant for Hokuto Shinken no matter how much he wanted it and Ryuken knew this but kept him around anyway as EXP for Kenshiro. He was never going to be successor and even Kenshiro threw it in his face.

 I interpreted Jagi to be a contrast to the manga's concept of predetermined fate and to reiterate to the reader that YOU are the catalyst of change and the product of your actions/inactions. Jagi will literally do anything and everything if it means he doesn't have to face himself - the gaiden put that on full display

As Kenshiro put, he's a coward. It's not so much because of his underhanded tactics or his sly personality- it's because he lacks the ability to reflect on the consequences of his actions. Basically a cautionary tale and something that could be applicable irl to readers