I've just finished watching this movie too and yes she was a German spy that was placed as a replacement of the real Marianne Beausejour who was executed by Germans in France. She was sent to operate in Morocco because nobody knew her there after she dismantled some French resistance groups. The matter is that everything is wrong with this character because she was too "French" for being a German spy, so "French" that she was able to give lessons of "Frenchness" to her "husband" ( that became on the way her real husband).

What I personally didn't get, is why he didn't want to kill her in the end. Supposedly he loved her, but I couldn't manage to figure out what exactly loved on her, because her whole existence was a lie. She is not Marianne Beausejour, we don't have a clue what her real name was and judging by how skilled fighter she was, she was probably responsible for uncountable deaths, plus the dismantling of the Parisian resistance groups. So what did he really loved on her after he realised the truth?

"Marianne" was most definitely a German spy who assumed the identity of the deceased (killed by Germans) French Resistance spy Marianne Beausejour. We are actually never told if she was a French woman who worked for/sympathized with the Germans or if she was a German woman with French ties who could pass as a Parisian. I tend to think she was a German national with fluency in French language and culture.

But what the film showed was that "Marianne", during the execution of her mission, actually fell in love with Max - for real. A key line in the film is when "Marianne" tells Max that she keeps the emotions real and that is the key to her success as a spy. But by keeping her emotions real, while playing the spy role of his loving wife, this results in her actually falling in love with Max - and he with her. In the scene where she and Max execute the German ambassador to France, if you notice, Max shoots the ambassador and several others whereas "Marianne" is seen just spraying the walls with bullets well above everyone. It is only when Claude (played by Xavier De Guillebon) grabs a pistol and points it at her that Marianne actually shoots and kills someone (having to do so to save her own life). Claude becomes an innocent casualty b/c he and Marianne are actually on the same side (the German side) but she had to kill him to sable her own life and not subvert the greater mission. And it is also key when right after Marianne shoots and kills Claude we see his (unarmed) wife, Monique, look at Marianne in horror/fear thinking she will also be shot. But Marianne doesn't kill her. At that point, Marianne just leaves with Max sparing Monique, a fellow German.

In my opinion, after she falls in love with Max, "Marianne" does want to just live a peaceful loving, life with Max and their child. But of course the Germans find her in London and threaten to kill their child if she does not assist them in spying on the Allies.

Max didn't want to kill her because he believed (very correctly imo) that she loved him and he definitely loved her. They had both killed countless people in their capacity as spies but their love for each other and their child was real.