Death in the city of light

I have studied European history and the Holocaust and been a consumer of true crime for many years, so I was surprised I had never heard of this case until a podcast (The Catalyst by Amber Hunt) covered it. It's a fascinating story, and thoroughly researched work of history, but the writing was uneven and disjointed at times. I was most disappointed with the epilogue. The author noted that the section was entirely speculative, so I wish he had actually speculated a lot more on the why, in addition to the how. What in Petiot's life may have turned him into a killer? Who was he working for? Why did the Gestapo release him from prison? Without evidence, it's impossible to come to definitive conclusions, but I would have appreciated educated guesses from the author who spent a significant amount of time with the primary sources for this case.