I think I developed thanatophobia this week
How did you develop Thanatophobia?
I'm frightened because of other people's fear of death
Having a fear of dying can cause stress and anxiety. If you're unable to take your mind off it then you should seek professional help. Otherwise you may have your daily life interrupted due to your fixation on this fear.
Humans cannot be all smiles and unicorns. People in reality get scared of what they don't know
Yes, but it should not consume your life.   Getting help might let you balance that.  If you are not comfortable getting help, learn more about death so that you better understand it.