Do you know that a Horror movie's objective is to bring out  physiological reaction, such as an elevated heartbeat?
Now that you say it, Why do people want to be scared, why are horror movies created anyway?
these movies overlap with the fantasy, supernatural fiction
I just don't understand why people want to go through it. When was the first ever horror movie created?
I'm not really sure, but I guess why these movies are made is  just to  evoke viewers' nightmares, fears, revulsions and terror of the unknown so that people can be better prepared 
Be better prepared for what though, a zombie invasion. Lol. Is there a big market for the horror movies in your opinion and which one was your favourite?
These movies usually would have an  intrusion of an evil force, event, or personage into the everyday world
What do you think the purpose of the evil force is? Is it to kill or to take possession and power or the living entity?
Im not sure, but you asked me when these movies started. I have an answer for you.Georges Méliès  was the first of such movie maker in the late 1890s