Xiao Feng had actually been thinking about this for a while. He was just worried that the four of them might have other bad intentions after taking them in, so he hadn't taken out the crystal core until now.
After this extended period of interaction, Xiao Feng realized that their inner natures weren't really that bad. They might have a bit of a sharp tongue, say some cheeky things, and act impulsively at times, but overall, they weren't malicious.
So, during the process of opening the vaults, Xiao Feng decided to give the crystal core to Bullet King.
When Xiao Feng first met the four of them, he was quite impressed by their abilities. For instance, the Poison King, Wang Xiao, had a rather disgusting yet powerful ability. He didn't need to put in much effort to train because all he needed to do was increase his level. He could become a perfect Poison King with his abilities.