OP here. This is very dark and powerful and well-written. I will admit, the "everybody's a creepy sociopathic rapist" is not my favorite kind of characterization for these characters, but it works here. (I'm more a fan of the original strips, where the characters have quirks, but none of them are evil.)

Although, for me, at least, the story works much better as horror than it does as porn. This is a good story, although an extremely uncomfortable one.

If you're looking for other dark prompts, well, these might fit the bill?

Any, Master/sex slave dynamic (au?), bestiality used as punishment
http://hetalia-kink.livejournal.com/17942.html?thread=57069334#t57069334

England/France, England raping and dominating France (with young America seeing it happen)
http://hetalia-kink.livejournal.com/10960.html?thread=22007504#t22007504

Any - noncon puppy play
http://hetalia-kink.livejournal.com/11411.html?thread=23971731#t23971731