The Good Buu was Mark's friend.
When Gohan started his rampage, Mark called for his friend - Buu, but was no match for Gohan's new powers, just as Buu wanted to release Evil Buu due to anger (again), Gohan killed him, saying "that's a payback for what you did to me you pink ugly blob", after that event Gohan mocked Mark, but left Bee untouched