People relate more easily to species that are similar in appearance to them and subconsciously consider anything that looks different as 'alien' and strange. This invokes feelings of fear and uncertainty, which repel people from insects.
Because of the way they are built, most insects move in jerky and fast moves. People feel uncomfortable around anything that moves too fast as it might remind them of an attacking predator. Jerky movements also seem as unnatural and perhaps robotic/mechanical.
Flying insects often produce unique and unsettling sounds which make people uncomfortable.
Some insects have very strange mating rituals such as the female eating her male partner after mating or baby offspring eating their mother from the inside out. This is quite unsettling for most people.
Most insects are small in size and some people are afraid that they will invade their body through their nose, ears or mouth openings. A study has shown that we actually eat many insects during our lifetime while sleeping.
Multiple legs, many eyes, expressionless faces, bright body colors and unfamiliar body features make insects look disgusting to many people. Because of these major differences in body shapes, we might feel discomfort near insects.
Many insects can poison us or inflict us with serious diseases. To increase our chances of survival, our brain motivates us to move away from them by using disgust (see also why do flies land on people).
Flies, for example, are often found on things that waste away such as carcasses, trash and excrement. Because of that, we associate them immediately with filth and negative things in our surrounding world.
The reason some people hate insects is that they unconsciously make them feel helpless. Because you can't expect the next move or the direction of an insect, you can easily feel helpless. Control freaks might fear insects.
In some countries people eat insects, while in some others the same insects are considered very disgusting. When a child sees his mother screaming at the sight of an insect the child might grow up finding insects fearful.
Horror happens when fear is combined with the inability to escape. An insect that flies inside your car, for example, can result in horror because you will be afraid and you won't be able to escape.
Insects are perceived by many people to be unclean and disgusting. They are usually seen in slums and poorly cleaned places. Sometimes these feelings of disgust come from the desire to protect one's social status by moving the person away from anything that is connected with lower classes.
It's as if they can look at you and see the danger in your angry or frightened eyes. Cockroaches may be stunned at the first sight of you, only to find the perfect timing while you try and grab the best thing to squish it with. And the fly goes back and forth from attacking you to running away.