Gustav, you had one job. And the task was failed successfully! It was impressive how many cliche red flags he raised. "After this one last job, I'll be an honest man!" XD
I think that little exchange when they wondered how to cover up the manslaughter pushed Tenma to say screw this, and go back to take it upon himself to get some justice, especially given how incompetent or corrupt any law enforcement he's seen has been.