From the way it seems, I've got a pretty disgusting sense of humour. I can laugh at and make jokes on things that people wouldn't dare to mention. My friends have told me that it's not normal and the internet just tells me it's an "early warning sign for dementia". I'm only realizing it now after I crossed the line with a friend and she told me that I'm not okay. I honestly don't know what to do anymore and I'm starting to get scared for and of myself. I don't know what I should do about it now.

whats on the internet is not always true
its normal to be apathetic

It is hazardous to self-diagnose oneself with a mental disorder and a dark sense of humor can be a quirk of personality, a consequence of your upbringing or a symptom of some other problem in equal measure.

I recommend you ask yourself the following:
1. Is your sense of humor causing you distress and/or severely affecting your ability to work in your environment (socially and otherwise)?

2. Does your family have a medical history of mental disorders? Is your current environment a regular source of (possibly immense and uncontrollable) stress?

3. How much control do you have over it? Do you feel that, societal expectations aside, you do not understand how you are "crossing the line"?

4. Have you considered that your friends might be being dramatic? Would you find it more reasonable to believe that they are simply not sharing that humor?

If you answered yes to 1. or 2, then I recommend you consider speaking of your issue with a trustworthy person (hopefully, someone more patient and wise than your friend and someone not directly responsible for your stress). If the situation does not improve in two months, consider seeing a counselor or similar professional.

The other questions are there to help you determine what steps you would like to take to change the situation (if at all).

Good luck.