People change, especially after experiencing life and death, their thoughts invariably alter.
First, Otto went through a serious illness. His surgery was successful, and he survived, but barring any surprises, his career as an assassin had come to an end. The moniker 'Shadowless' officially transitioned from his code name to that of an assassin organization.
Therefore, Otto's mindset began to shift. The joy of surviving was gradually fading, and he started to feel distressed about being confined to the small hospital room.
Sara could sense a change in Otto's attitude, which is why she hoped Gao Guang could persuade Otto to find some tasks to occupy his time.
So Gao Guang had to perform a display of deep mentor-student affection with Otto, but once Sara was gone, and it was just the two of them, the atmosphere was not so pleasant anymore.
After a moment of silence and cordiality, Otto finally said, "You used my girlfriend as your stepping stone…"