A big part of Kuma's character is the fact that he takes pain on the behalf of others
So why exactly was Kuma killed?
If you put two and two together, the answer is simple. Kuma died because he took the punishment that was meant for Bonney. Again go back to the pages I posted earlier, Kuma literally takes suffering on behalf of others. With the pain, he literally absorbed their pain. And so with the situation with Bonney, I don't think it is so simple as the WG merely held Ginny or Bonney hostage and forced Kuma to become a robot. No, I think Kuma literally took Bonney's position. The WG wanted to make Bonney a pacifista, but then Kuma volunteered himself so that Bonney could be free.
This makes a lot of sense when you see stuff like this:
Saturn: "We no longer have any use for her"
So I guess building off of this, I want to posit a theory.
I think that Ginny is Big Mom's sister.
Her shirt has the word sister on it, but why? She's definitely not Kuma's sister that's for sure. I guess she called Ivankov big bro, but to what degree is that relationship even relevant?
At this time, Ginny is 21 years old. And this scene happens 30 years ago. Meaning Ginny would be 51 at the moment, only 17 years younger than Linlin. Hence I think its possible that Ginny is Linlin's sister.
They both got freckles on their cheeks, an appetite, and probably pink hair.
I think that after BM's parents ditched her, eventually one of them had another kid, Ginny. This is why Ginny was enslaved. When Ginny was 4 years old(the age she said she was enslaved), BM would have been 21, she would have been a notorious pirate at this point. The WG, seeing what BM's blood was capable of, likely enslaved her parents(and any siblings she might have had), as they all had the potential of creating another monster like BM.
Eventually MADs appeared and with the ability to clone and manipulate lineage factors, the WG likely wanted to harness the power of BM's blood. This is probably why Bonney was created, to be a weapon for the WG.
"But Bonney is a fodder"
Bonney is one of the supernova, yet by all evidence, she is still a child. You can see this from Saturn's words, Vegapunk's, and the sheer fact that there is no mention of her despite us being 14 years away from the main storyline in Kuma's flashback. If Bonney is just 10 or 12 years old and she already has a bounty of over 300 mil, then she ain't just a fodder, she's a prodigy. Just like how Ginny was shown to be a prodigy(she could wiretap).
Look at what Akainu said to Bonney. Why would Akainu be afraid if Bonney escaped? I think her being a candidate for pacifista with Big Mom's blood would make this line make a lot of sense.
Bonney has high potential. She is a young child with weak trio level fighting capabilities. By the time she is truly an adult in her prime, she could legitimately be a top tier. I think the WG saw that and wanted to turn her into a pacifista.
Kuma stepped in and offered to be the pacifista instead of Bonney. That's why Vegapunk wasn't allowed to tell Bonney what happened to Kuma, he literally took Bonney's place. Anyways that's the theory.
TlDr


So why exactly was Kuma killed?

If you put two and two together, the answer is simple. Kuma died because he took the punishment that was meant for Bonney. Again go back to the pages I posted earlier, Kuma literally takes suffering on behalf of others. With the pain, he literally absorbed their pain. And so with the situation with Bonney, I don't think it is so simple as the WG merely held Ginny or Bonney hostage and forced Kuma to become a robot. No, I think Kuma literally took Bonney's position. The WG wanted to make Bonney a pacifista, but then Kuma volunteered himself so that Bonney could be free.
This makes a lot of sense when you see stuff like this:

Saturn: "We no longer have any use for her"
So I guess building off of this, I want to posit a theory.
I think that Ginny is Big Mom's sister.

Her shirt has the word sister on it, but why? She's definitely not Kuma's sister that's for sure. I guess she called Ivankov big bro, but to what degree is that relationship even relevant?
At this time, Ginny is 21 years old. And this scene happens 30 years ago. Meaning Ginny would be 51 at the moment, only 17 years younger than Linlin. Hence I think its possible that Ginny is Linlin's sister.

They both got freckles on their cheeks, an appetite, and probably pink hair.
I think that after BM's parents ditched her, eventually one of them had another kid, Ginny. This is why Ginny was enslaved. When Ginny was 4 years old(the age she said she was enslaved), BM would have been 21, she would have been a notorious pirate at this point. The WG, seeing what BM's blood was capable of, likely enslaved her parents(and any siblings she might have had), as they all had the potential of creating another monster like BM.
Eventually MADs appeared and with the ability to clone and manipulate lineage factors, the WG likely wanted to harness the power of BM's blood. This is probably why Bonney was created, to be a weapon for the WG.
"But Bonney is a fodder"
Bonney is one of the supernova, yet by all evidence, she is still a child. You can see this from Saturn's words, Vegapunk's, and the sheer fact that there is no mention of her despite us being 14 years away from the main storyline in Kuma's flashback. If Bonney is just 10 or 12 years old and she already has a bounty of over 300 mil, then she ain't just a fodder, she's a prodigy. Just like how Ginny was shown to be a prodigy(she could wiretap).

Look at what Akainu said to Bonney. Why would Akainu be afraid if Bonney escaped? I think her being a candidate for pacifista with Big Mom's blood would make this line make a lot of sense.
Bonney has high potential. She is a young child with weak trio level fighting capabilities. By the time she is truly an adult in her prime, she could legitimately be a top tier. I think the WG saw that and wanted to turn her into a pacifista.
Kuma stepped in and offered to be the pacifista instead of Bonney. That's why Vegapunk wasn't allowed to tell Bonney what happened to Kuma, he literally took Bonney's place. Anyways that's the theory.
TlDr
- Ginny is BM's sister. She was enslaved because of her blood, just like Kuma
- Bonney was created from Ginny, either as a clone or some sort of forced pregnancy, for the purposes of trying to harness the power of BM's blood
- Bonney is a young child and the strength she's shown despite being a child means that she is a prodigy. The WG saw her potential and wanted to use her as the pacifista
- Kuma decided to be the pacifista instead, taking all of bonney's suffering upon himself so that she could be free