At the end of the battle, Buccellati attacked Pesci repeatedly and suddenly Pesci was ripped apart into pieces. How could this happen?
Sticky Fingers' ability is to create and remotely control zippers on the object it makes contact with its fists. It can also create a zipper all the way around (for example) someone's wrist so that when it is oppened it is essentially ripped from the body. This is how it rips things apart into pieces. During the manga we see Bucciarati use his stand like this multiple times.
How does Sticky Fingers' ability work toward Pesci in Golden Wind?
Bucciarati can choose if the pieces are still connected or not (e.g. if the wrist is still circulating blood). This is not explicitly stated but we can figure this out from these two instances:
Bucciarati seperated Mario Zucchero's head earlier in the manga with seemingly no effect on Mario's head. Pieces are still connected. ¹
Bucciarati also seperated his own body into parts earlier in the same arc which stopped his blood circulation. Pieces are disconnected. ²
At the final battle, Pesci threatened Bucciarati with killing the turtle. This caused Bucciarati to see him as "nothing but the soul of an ordinary scumbag". ³
He chose the second option without caring if Pesci is dead or not when he ripped him apart. We can clearly see him splitting his head in two, causing serious brain damage. This kills Pesci.
1: JoJo Chapter 462 | Golden Wind chapter 23
2: JoJo Chapter 497 | Golden Wind chapter 58
3: JoJo Chapter 499 | Golden Wind chapter 60