Don't be. If someone doesn't like the premise of your story, then they have no business picking it up in the first place. I have read a number of novels with darker subjects and they have stuck with me years later. One book was about abuse, but it was NOT disappointing; and it was of the ones I liked most. A friend from high school wrote a school picture prompt about a girl who woke up to find she was raped. I found that to be bold and wish I had thought of it.
I have felt that way at least once and I find myself having to force bad things on characters and wondering if I am being too nice to them sometimes. I remember telling my mother about an idea of mine that I came up with about killing off a teen character at the end that was central to my story. His death occurring was the whole point of the story. She said that was horrible and instead I should write about my main character befriending a dolphin instead. I looked at her and responded with "No way and now I'm definitely gonna kill him off." I asked my sister her opinion and she loved the idea. Even now, I still find myself wanting him to live, because he was made to be an extremely likable character. But if I do, it weakens the whole story and looses it's impact. I feel at that point why bother even writing it if he doesn't.
Writing can be art for some people; a way to express ourselves and write about emotional and intense subjects that mean something to us. The reason their are so many books about these things is because they exist. If they didn't exist there wouldn't be any about it, right? Some books come out for the better when they put characters in horrendous situations. It can add depth and make it more real and interesting. If done right and in good taste, it can be a brilliant creative move. So it comes down to how you feel about it. Are you in love with the idea? It is better creatively to keep it in or nix it? If you think it is best to keep it in ignore what others think. Go for it? And if you do build up the courage to tell others, and they don't like the the fact that it has a dark theme, they don't have to read it. But that doesn't mean others won't want to or be intrigued by it. You're not an evil sicko, you just want to write something emotional and profound.
Hope this helps and happy writing!