My theory is that he was shaping her up to be come his next worthy opponent he always wanted.
Abuse can't break CoC user's spiri, it only makes them stronger, kaido knows that, and he also knows that yamato heats him, so he forced her to take a similar experience to his, the beatings, imprisonment and the starving is but a drop of water compared to what kaido probably went through.
Yamato will become stronger through hardship, and eventually give kaido an exciting fight.