I think that if you think bad/morbid/unwanted thoughts but can push them out of your mind and think about other things, it can be normal.
If, however, you can't rid yourself of the thoughts, like your mind is a broken record that keeps repeating and repeating them whether you like it or not, it could be an obsessive disorder. I was diagnosed with obsessive thought disorder a few years ago. It is awful.
When I was diagnosed, the psychiatrist prescribed me a high dose of antidepressants, which is supposed to help break the cycle and reprogram the brain. I never took the meds (was breastfeeding a newborn and was also scared of the side effects). At that time, just talking to the psychiatrist helped a lot.
But I'm thinking of going again because I'm having the same problem lately.
. It's hard.