A childhood wound: at the funeral of the officer who died protecting him, her husband said something cruel to Louis.
Louis said something weird to Carl about a cop's husband that Carl couldn't parse — he first assumed it meant Frank and Maggie My, but Louis had never had dealings with them. Katia slipped in beside him on the transport platform, wrapped her arms around her legs, and explained. When Louis was young, a police officer was shot and died protecting him. At the funeral for that officer, her husband said something very cruel to Louis, and it stuck with him ever since.
Princess Donut cut the story off from Mongo's back, shouting that nobody should tell sad stories about Louis's past because it would jinx the rescue.
The formative trauma behind Louis's cryptic remark to Carl.