The order was to kill.
Kuma is a WG dog, in that arc moria said that kuma follows the WG orders to the letter.
He had no reason to honor zoro's request.
For him to be satisfied with only torturing zoro that means that he aknoledges zoro's valeu as equal to luffy.
Kuma chose zoro as his first opponents and zoro persevered till the end, he couldn't fight physically but he suggested another sulotion to offer his life instead of luffy, kuma didn't just kill zoro swiftly he showed him hell. And huma was sure that zoro would die judging by his previous injuries, but zoro survived.
This is entirely wrong. Zoro himself stated with future accomplishment, not his own, meaning it´s something he would essentially become and gives up now for the current Luffy.
And once again, Kuma underlines Luffy´s head for the WG would be enough, Zoro´s never was.
Kuma never did that, he acknowledged hurting Luffy after Zoro´s speech would be dishonorable.
Luffy´s head
Zoro himself admits his head is not enough, hence argues with being future WSS. Also admits it´s a loss for Kuma, only that since he will become WSS someday, it´s not too much of a loss, still a loss regardless.
Both Zoro and Kuma say Kuma not killing Luffy after this is a matter of honor, not replacement of Zoro. If Zoro´s was enough, there would be no meaning to torture him, just kill him and present his head to WG.
Later on Shabondy Zoro himself states he survived due to Kuma´s mercy, meaning he knew Kuma never meant to kill him.