It doesn't matter, in the end, what Yuugi's response would have been. Because Kaiba's response is to peel the paper back, glare at the shitty little baked good in his hand, and take a bite out of it.

He's committed. He's not sorry. He's going to eat it, so that Yuugi doesn't try and do it himself.

He'll accept the consequences. Surely the scientists wouldn't try to kill him, right? Death is not the best vehicle for attaining Light. There are better ways to extract suffering.

Such as a slow-acting, non-lethal poison.

...Right?