We often hear that artificial intelligence may harm or even kill humans, so it might prove dangerous.

How could artificial intelligence harm us?

tl;dr

There are many valid reasons why people might fear (or better be concerned about) AI, not all involve robots and apocalyptic scenarios. 

To better illustrate these concerns, I'll try to split them into three categories.

Conscious AI

This is the type of AI that your question is referring to. A super-intelligent conscious AI that will destroy/enslave humanity. This is mostly brought to us by science-fiction. Some notable Hollywood examples are "The terminator", "The Matrix", "Age of Ultron". The most influential novels were written by Isaac Asimov and are referred to as the "Robot series" (which includes "I, robot", which was also adapted as a movie).

The basic premise under most of these works are that AI will evolve to a point where it becomes conscious and will surpass humans in intelligence. While Hollywood movies mainly focus on the robots and the battle between them and humans, not enough emphasis is given to the actual AI (i.e. the "brain" controlling them). As a side note, because of the narrative, this AI is usually portrayed as supercomputer controlling everything (so that the protagonists have a specific target). Not enough exploration has been made on "ambiguous intelligence" (which I think is more realistic).

In the real world, AI is focused on solving specific tasks! An AI agent that is capable of solving problems from different domains (e.g. understanding speech and processing images and driving and ... - like humans are) is referred to as General Artificial Intelligence and is required for AI being able to "think" and become conscious.  

Realistically, we are a loooooooong way from General Artificial Intelligence! That being said there is no evidence on why this can't be achieved in the future. So currently, even if we are still in the infancy of AI, we have no reason to believe that AI won't evolve to a point where it is more intelligent than humans.

Using AI with malicious intent

Even though an AI conquering the world is a long way from happening there are several reasons to be concerned with AI today, that don't involve robots!
The second category I want to focus a bit more on is several malicious uses of today's AI. 

I'll focus only on AI applications that are available today. Some examples of AI that can be used for malicious intent:


DeepFake: a technique for imposing someones face on an image a video of another person. This has gained popularity recently with celebrity porn and can be used to generate fake news and hoaxes. Sources: 1, 2, 3
With the use of mass surveillance systems and facial recognition software capable of recognizing millions of faces per second, AI can be used for mass surveillance. Even though when we think of mass surveillance we think of China, many western cities like London, Atlanta and Berlin are among the most-surveilled cities in the world. China has taken things a step further by adopting the social credit system, an evaluation system for civilians which seems to be taken straight out of the pages of George Orwell's 1984.
Influencing people through social media. Aside from recognizing user's tastes with the goal of targeted marketing and add placements (a common practice by many internet companies), AI can be used malisciously to influence people's voting (among other things). Sources: 1, 2, 3. 
Hacking.
Military applications, e.g. drone attacks, missile targeting systems.


Adverse effects of AI

This category is pretty subjective, but the development of AI might carry some adverse side-effects. The distinction between this category and the previous is that these effects, while harmful, aren't done intentionally; rather they occur with the development of AI. Some examples are:


Jobs becoming redundant. As AI becomes better, many jobs will be replaced by AI. Unfortunately there are not many things that can be done about this, as most technological developments have this side-effect (e.g. agricultural machinery caused many farmers to lose their jobs, automation replaced many factory workers, computers did the same). 
Reinforcing the bias in our data. This is a very interesting category, as AI (and especially Neural Networks) are only as good as the data they are trained on and have a tendency of perpetuating and even enhancing different forms of social biases, already existing in the data. There are many examples of networks exhibiting racist and sexist behavior. Sources: 1, 2, 3, 4.

