What Animefanka said. You need to be logged in to view mature pages/episodes, so non-Tapas members reading your comic will most likely skip those ones. It's good to have your comic up on other sites, ones without the same restriction, so those folks can be directed there. I follow your comic on Comicfury, so you could always advertise that mirror via little end-cards or something, letting readers know they can go there to read everything unrestricted.
Edit: Totally missed that last question. You can put up a warning at the beginning of you want, but it's up to you. Your comic being a horror it's only natural that it would tackle darker subject matters, there aren't a whole lot of horror stories out there that don't, so your audience should expect that as being part of the genre. Also putting up that kind of warning has it's pros and cons. I had a disclaimer at the beginning of mine for a while, then decided to take it off and keep the warning in the summary, the views on my comic went up by a lot and I got a few reader out of people who wouldn't have check it out if I had kept up the disclaimer. So it's a mixed bag, you can either take extra precautions and scare potential readers away, or keep it to the summary and let people fully take their reading experience into their own hands.
Again, I personally would expect a story in your genre to be a darker read, but it's up to you. People nowadays definitely are more sensitive about fiction than they used to be, so the extra precaution isn't totally unsound.
