That is exactly the problem though…
Blackbeard‘s just a rival pirate at the end of the day while Imu oppressed the world for 900 years. Who is thematically and logically the bigger bad here?
Blackbeard’s new goal of taking over the world? — Imu’s been already doing that for the last millenium, probably even longer than that if he was also responsible for the first world’s collapse.
Thing is you are seeing the present situation.
Everything ties back to Ancient kingdom.
We have some fine hints which points to Ancient kingdom might not be as good as we think.
Have you ever wondered why Joyboy even sailed out to the seas to become pirate?
BB and Xebec both shared one goal - King of the world. BB is the only pirate who eyes at IMU's throne.
If I have to guess then Ancient kingdom was once ruled by evil king - BB is descendant or reincarnation of him while Luffy is reincarnation of Joyboy.
There is a reason why Oda depicted BB as polar opposite of Luffy - in all sense - goal, ideals, character, aspirations, crew and even thematically - sun (light) vs darkness
Have you wondered why IMU marked BB - only villain to be marked alongside MC?
Their rivalry would go back to ancient kingdom time.