If Kuma was actually as brutal as his reputation says, then why would he just leave after causing some pain to Zoro? Also, it seems that Zoro is extremely pissed off at the end of the chapter, which makes me think that he either couldn't take all of the pain from Luffy's body or he got pitied by Kuma.
@Urian:
This scene is just paralel than the one that we saw when Zoro fought against Mihawk, putting Luffy´s pain and fatigue in his body is the way how Zoro will learn that even him, the second one more powerful in the crew, isn´t ready for the new world and he must do an huge step.
You're probably right about this part.
@Urian:
Now about Kuma, he seems to be the most powerful Shichibukai that we have seen until now, the other three seems are normal human beings with broken devil fruit habilities, Kuma is just a damn robot with a physical strenght beyond the others.
I believe that Kuma was a normal human nicknamed Kuma "The Tyrant" who ate the paw paw fruit until Moria defeated him some time ago, yes I have said Moria.
With this move Moria became a Shichibukai and stole Kuma shadow but he knew that if Kuma original body dies then the shadow will be lost and this is when Vegapunk enters in the scene, Vegapunk makes the new Kuma for body enhacing the old body and Moria returned the shadow, since the original owner of the shadow was the own Kuma then he ressurected more powerful than before but with his own force of will.
With this Kuma became another Shichibukai and Moria started to prefer zombies instead of modified human beings as his soldiers.
There's really no evidence that Kuma is the strongest Shichibukai. Yeah, he may be a cyborg, but that doesn't count for much in the OP world. Plus, we still haven't seen the fighting strength of Jimbei or the final unnamed Shichibukai.
I doubt that Moria defeated Kuma. What is seems like to me is that Kuma was defeated by someone very high up in the WG when he was still a pirate, most likely one of the 3 admirals, and got most of his body destroyed. Instead of being taken to Impel Down and executed, he may have made a deal with the WG to save his life (maybe he was so destroyed by the battle that Dr. Vegapunk had to make him a Cyborg to keep him alive) and ultimately become a weapon for the WG, which would explain why he is so obedient to the WG.