Yeah him burning up the herb made it pretty obvious he was just an asshole.
Him burning the herb made prefect sense to me.
He was an outsider regardless of his claim to the throne and the herb makes him the strongest one around. Who's to say that given the radical nature of his power grab and the sudden change in actions with the outside world that some others wouldn't think "Fuck tradition, it's up to ME to right this wrong!"? To him he needed to keep that power status.
He could have an uprising on his hands in a matter of days or weeks, having to fight off 4-5-10-15 or w/e number of people who got BP powers from the herb after a break in or with insider help. They stated it in the movie, that it's a colonization/ military tactic when taking over.
As for what he wanted, I think he did want to do this for the betterment of oppressed black folk around the world. He was a parallel to
Nakia, she pointed out how in Wakanda they have so much yet just outside the boarders people that look like them were struggling and living in fear. She wanted them to become more involved, through charity and relief efforts IIRC. Erik wanted to empower people like them by using Wakanda weapons to cause revolution through violence, give black people around the world the power to topple unjust governments/ systems through pure fire power. (GET OUT even talked about how the world's a mess and Wakanda is THE BEST and if told they could "clean up" this mess easily and I remember it being pointed out that the tech advantage that keeps Wakanda safe and comfy is gonna go away not too far into the future with the rate at which the outside world is advancing).
BP learns from both Erik and Nakia. By opening boarders with the world and sharing their knowledge, to lead the world going forward and help others who look like them by doing so through black areas like Oakland ... empowering them through these efforts.