I struggle with this a lot too because I write for young adult, but I also write fairly dark material. In the end, it really depends on your target audience and what your intent is. Is there ever a line that crosses too dark? Perhaps, but then maybe your story isn't what you originally thought it was.
In your example, you say that you "knocked off a dog" in one of your romances. Well, how did it happen? Did the dog die because it was hit by a car in the tragic heroine's lowest point of the story? Or did the abusive lover kill the dog to set an example for the heroine? There's a huge difference between the two. The first one can still be in the realm of romance and is sad but not too dark, while the second one... well, I don't think that can be considered romance any more because it seems too dark and more like a thriller or something. Does that make sense?
There was a scene in my own book where the protagonist originally was going to do something that I later realised was very twisted for a teenage girl to agree to. I liked the idea and thought it was totally messed up, but it was just too dark for the young adult feel I was going for. The idea ended up working to my advantage since I could have her find a different way while shunning the disturbing way. Doing so fit the character and theme much better. If I had been writing an adult book, maybe I could have gotten away with it, but for the crowd I am writing for, it was better to have it dark but maybe less morally ambiguous.