Kaido has been an enigmatic character since he appeared. We know that he believes that the death completes a person, so it’s reasonable to guess that he’s concerned with his legacy. As the Kaido-Yamato battle has continued, it has become clear that Kaido does not take Yamato lightly, so I believe that Kaido may, despite everything, consider Yamato his heir.
Kaido has been training Yamato.
Chapter 1024 showed Kaido say that, if Yamato insists on calling herself Oden, it’d be better if she did die. However, it’s been 20 years of this, Yamato has lost to Kaido countless times, and yet he hasn’t killed her. The easiest explanation is that he wants Yamato to be as strong as possible. Even now, as they battle, he’s trying to kill Yamato because he knows she can only reach the peak of her power by being pushed to the limit.
Kaido respects Yamato (in his own way).
The question of why Kaido would refer to his daughter as his son has been a driving force in the Yamato gender debate, as someone willing to imprison their child doesn’t seem like the type to respect their pronouns. People seem to have settled on the idea that Kaido really wanted a son, but the answer could be even simpler: Kaido respects Yamato’s decisions completely. He respected her decision to use the name Oden enough that he treated her as an enemy, imprisoned her for 20 years, and is now willing to fight her to the death. Compared to these, using the word “son” is nothing.
Kaido probably failed in his original dream.
The details here I can only speculate at, but I believe Kaido’s life long goal has involved the defeat of the Celestial Dragons. Roger and Garp defeated Rocks at God Valley while defending Celestial Dragons, so it’s natural to suspect that the Rocks crew opposed the Celestial Dragons. Kaido’s current plan to create a “world of violence” may not seem related, but the Celestial Dragons/World Government are the only what maintain order in the world, so naturally they would need to be defeated. Kaido’s knowledge of Joyboy makes me suspect that he is aware that the coming war will be led by the next generation, and his depression comes from realizing he won’t be a part of it. However, he has created an army that respects strength, and an heir with immense strength, both of which will likely participate in the coming war.
Yamato may carry Kaido’s will.
One of Yamato’s deepest desires is to be free. Kaido’s subjugation of Wano and imprisonment of Yamato mirror the Celestial Dragons subjugation of the world and use of slaves. Yamato could very well be inspired to fight against the Celestial Dragons because of this, thus inheriting Kaido’s will.
“There are no easy answers”.
Kaido likely took over Wano because he was aware that it would play a role in the coming war, and trained Yamato with the goal of making her as strong as she can be for the fight to come. If this is the case, the answer to Yamato’s question would be: he didn’t really steal her freedom, he respected her free choice to be Oden, and imprisonment is one consequence of that. Even now, he says “If you want to wear the name Oden, you must be ready for war.”
If Kaido views Yamato as an important part of his Legacy, we may come to understand Kaido partially through seeing what he hopes Yamato can accomplish. Also, many people note that Yamato’s desire to go out to sea is a red herring. This could be true, but a red herring doesn’t mean that the opposite of a statement will happen, it means that the existence of a question distracts from what’s really important. The question of whether Yamato will go to sea or become a guardian of Wano could simply be a distraction from Yamato’s real conflict: whether she can accept Kaido as a father.
Theory by Roostalol (https://www.reddit.com/user/Roostalol).