Until the moment where Zoro showed his willpower to protect everyone, Kuma was an enemy. In fact Kuma was about to catch Luffy until Zoro managed to hurt him. And it's not that it was easy with him either, because he fought against Zoro, he launched an ultra-powerful attack that knocked everyone out, he put a test on Zoro to fulfill his deal.
Precisely, if Kuma had been an ally at that time, he would have acted in a similar way to Sabaody, saving their ass and not putting them in more trouble.
Luffy will be the son of Dragon, but had it not been for Zoro demonstrating his willpower, which in turn serves to demonstrate Luffy's potential, Kuma would have captured him without hesitation.