Kyoshiro and Sasaki are friends
That's why he didn't kill
That's the most logical explanation
Denjiro is not the type of person to kill. He killed Hiyori to fake her death. kyoshiro also seems to be an intelligent person. Look how he brought lot of reinforcements to help Kinemon and judge the location of the port.
Moreover, they are in a battle in a territory with 2 yonko. Just 1 yonko, Luffy and his crews ran away from WCI now they have to face 2 yonkos. Brute strength alone won't help them here given the enemy has number advantage as strength advantage as well.
Denjiro, a smart person to know the real location of the port meeting location might want to interrogate Sasaki so he can get intels on Beast and Big Mom Pirates as many as he wants. Even little intel is valuable since the enemy has number and strength advantage.
Other possibility is Sasaki might be used as hostage or Jinbei might have history with Sasaki (both of them are fishman) so Oda might make Jinbei turn Sasaki into ally after Jinbei defeats him in an intense battle.
Turning every captured enemy into ally, interrogating captured enemy for intels and using captured enemy as hostage. The more forces with more disadvantage do that the more likely forces with disadvantage might have a chance to defeat forces with more number and strength.