Sympathetic to nazi-Germany?

I didn't think this was sympathetic. I do think it presents things in a more rounded fashion without needing to rely on caricatures. What it did do, which you see in some films (like Polanski's Pianist) is the distinction between the armed forces and the Third Reich. The tension between the two is illustrated in this production between Hilda and Hartenstein and between characters in the German HQ that Hartenstein messages.

Re-the gentlemen bit, that's commented directly upon by Hilda. She tells them they're not gentlemen for the numbers they killed including her baby. I recommend a re-watch to listen to what is being said between characters and the message coming from that. Hartenstein's unusual response comes from guilt after realising his error at bombing a boat with civilians. He realises it was wrong.

rouge silk,
fierce concentrated joy,
fires the blood

It's strange that you are not sure why this is. It is because that is what actually happened, this is not fiction by some neonazi apologetics.

As the previous replyer mentioned, you fail to make the distinction between the Nazi government and the armed forces (not including the Waffen-SS). No doubt because of quite a few films and series being made showing the German forces as pure evil and full-embodied jew hating master race killers, which is blatantly an insult to truth and reason.

If you read books by, or would have been to testimonies of torpedoing survivors, you would see that they held the German navy in regard.

Well, it is a story of one incident and focussed on one U-boat commander and crew. Here's some conjecture on my part, however, and I don't know what Lt. Hartenstein thought/saw of the result of the almost 20 previous "kills" of the U-156. Could he have started to have a "change of heart" upon seeing the horrible effects first hand of 20th century "push-button" warfare, where civilians get little warning or chance to get out of harm's way? A far cry from 19th and earlier, when armies/navies faced each other in pitched battles on open seas or fields. The U-156 only made one other successful attack in '42. Do we know how/why they ended up in the Caribbean, presumably on orders from high command, where they were sunk in early '43? Once there, could Hartenstein have just been trying to sit it out, or after realizing the horrible consequences of his actions, let fate decide if the U-156 would survive or be sunk with all hands? Presumably, the crew may not have agreed to surrender at that stage, although they should have realized they would lose, with the U.S.A. having gone directly into the war. His portrait gives the impression that he was a gentle soul. BTW as a kid growing up during and after WWII, remember we had a captured German sub on exhibit at the port of Montreal, as they did come into the St. Lawrence estuary. It seemed rather small.

First time German submarine war started during WWI as counter measure against the British Blockade because that didn't spare contraband (food) against international law.
Maybe You forgot that the British Airforce prepared for unrestricted bomb war against civilian targets since the 1920's. A long time before WWII British military and political leaders stated that starving and bombing civilians were the proper conduct of war for Britain.