I currently have a fantasy novel out in print and Kindle format. A lot of people have followed me because of my fantasy book, as well as my comments regarding fantasy. I started writing because of my love for fantasy. Most people probably still think of me as a "fantasy author."
Why, then, did I start to dabble in horror? Why have I published a horror novelette, and why do I have a horror novella on the way? A while ago I saw a graph showing the popularity of various book genres. Amazingly enough, horror was listed near the bottom, while fantasy was near the top. The horror niche, while popular, isn't considered popular enough in terms of marketing. Certainly a lot more people have bought Gods on the Mountain than they have Raw Head, even though the latter is cheaper than the former. Clearly, if I want to make money, I should probably push more into the fantasy genre than horror.
The truth is, the reason I got into horror was I wanted to explore ideas and concepts I couldn't with fantasy. I wanted to explore supernatural and perilous elements that would be harder (though not impossible) to do with fantasy. It likewise permits me to explore stories set in our own world. Plus (if I may be excused for using a ten-year old's argument), I just felt like writing in horror.
The wonderful about being an author (especially an indie author) is that you can adapt your writing focus. You're not bound to write this or that. Granted, this can be seen within reason: if your fans want you to write a sequel to a popular book, it might be good to write said sequel; if you're seen as a Christian author, writing violent erotica might shock your fanbase and loose you readers. However, for the most you are completely free to explore and develop your style however you see fit.
And, frankly, authors should be permitted to do so, so long as they do it without looking down on their fanbase or forsaking those who supported and loved them during the early days.