Boa has a concise backstory in a 2-volume arc. Her current arrogance is linked to her looks and to her humiliation, of which the tattoo is a reminder. Then we're shown where she got her tattoo, linking her story to the darker sides of the WG and to Fisher Tiger. It works because it's all that's needed to humanize her and to show that she is on Luffy's side against Ace's captors.
Kaido gets an equally-short flashback in 15-volume arc. He was a child soldier and was experimented on by the WG. Neither info tells us anything new on either him nor the world. His relationship with King is kept to a minimum, his time as a Rocks pirate mostly skipped. The flashback is hollow. It doesn't help us change our minds on Kaido, nor to better understand why he became a tyrant.
A short, fast-paced flashback like Boa's would've been fine, if we were given something interesting to work with. Instead, Kaido was given the most basic bitch miserable childhood and that's it, no greater events in the OP lore, no deeper motivation.
Kaido gets an equally-short flashback in 15-volume arc. He was a child soldier and was experimented on by the WG. Neither info tells us anything new on either him nor the world. His relationship with King is kept to a minimum, his time as a Rocks pirate mostly skipped. The flashback is hollow. It doesn't help us change our minds on Kaido, nor to better understand why he became a tyrant.
A short, fast-paced flashback like Boa's would've been fine, if we were given something interesting to work with. Instead, Kaido was given the most basic bitch miserable childhood and that's it, no greater events in the OP lore, no deeper motivation.