Why didn't the border killer 2 boy kill him?

Because the boy has a kind heart and integrity, he hopes to return Alessandro's kindness by training to be a border guard.

1. When Alessandro went home after the gang solved Alessandro's problem, there was a scene where the little boy jumped out of the car and didn't want to be with this group of people, which showed that the little boy was still good in nature.

As an experienced veteran, Alessandro is very grateful. A year later, he found the little boy in a hotel and wanted to repay Alessandro by training him to be a border guard.