Did Kizaru actually hoped for the Straw Hats to successfully rescue Vegapunk and pulled a Garp on Marineford? At the very least, I think he didn't want to finish VP himself so if they're taken out by St. Saturn it'll be his worst case scenario.
Speaking of Saturn, he's evil but he's also logically sound. He knew the Buccaneers were one of the last believers of the Sun G** so lobotomizing him and ensuring his bloodline ends here while also making use of his abilities as their personal soulless Warlord was the perfect showcase of his title as the G** Warrior of Science and Defense. That's some top-tier handling of a negotiation and confidential matters.