Early Career and Bertinelli Massacre
As part of his work for Galante, he participated in the Bertinelli Massacre, the slaughter of almost the entire Bertinelli Crime Family to allow Galante to become the dominant mafia don in Gotham City (though he was financed and supported by Black Mask's Criminal Organization). Though the assault was successful, it would ultimately prove Happy's undoing, as one member of the family, the young daughter Helena Bertinelli, was spared.
Encountering Harley Quinn and Death
Years later, Happy encountered Quinn in a back alley of Gotham City. Having heard that she was no longer under Joker's protection, he was looking forward to beating her to death, slowly, with a crowbar. Before he could strike, he was shot in the throat with an arrow from Helena Bertinelli's crossbow. Happy was the third man responsible for the massacre whom Helena had killed, after Galante and Carlo Rossi, and leaving only Victor Zsasz. 
This article section is incomplete and requires expansion.
- Shotgun: Happy used a shotgun to kill the Bertinelli crime family.
- Crowbar: Happy was going to use a crowbar to beat Harley Quinn to death.
Behind the scenes
- Hodson, Christina (writer) & Yan, Cathy (director) (February 7, 2020). Birds of Prey (And the Fantabulous Emancipation of One Harley Quinn).