Q:

Why is "evil" such a taboo word in English conversations?

evil (noun):
[uncountable] a force that causes bad things to happen; morally bad
  behaviour.
evil (adjective):
having a harmful effect on people; morally bad
extremely unpleasant

(OLD)
That's a very readable definition with a very wide range of applicability. Seemingly, anybody who takes actions (intentionally) that lead to bad things, can be deemed evil. I have looked at other definitions, and the pattern holds: it's just a term to describe people who do morally bad stuff.
And yet, whenever I use the word 'evil' (to describe someone, i.e. "they are evil") in a discussion with natural English speakers, I am almost always deemed unserious and immature, and my remark inflammatory and inappropriate... that's to be expected at times, but the issue is that this happens even if the others agree that the person in question does morally bad things!
I have noticed that if I use a word other than evil, but still with the same definition, I am not going to get into any sort of trouble. For example if I said

They are amoral.

or

They are cruel.

nobody would bat an eye. Yet the word "evil" (which most dictionaries list as a possible synonym to "amoral" and "cruel") makes everybody lose their mind.
Why are these people so scared of using their own words?

A:

You have asked a very pertinent question. Since the strong connotation of 'extreme physical or moral wickedness' has already been established by a senior member in the earlier comments, I shall not go further with the idea that the word 'evil' is employed very sparingly by native speakers because they think it ought to be used only for extremely dire situations; but I should like to highlight two other aspects that might possibly lead to its avoidance by native English speakers.
(1) Strong religious connotations:
Most religions are founded on the fundamental opposition between Good and Evil. Since the word 'evil' thus has strong religious connotations, many native speakers might hesitate to use it in everyday situations, not least because the ordinary human cruelties, although tragic and very unfortunate, cannot really be equated with Evil on a cosmic scale.
Recommended reading: https://en.m.wikipedia.org/wiki/Good_and_evil
(2) Modern liberal attitude to human behavior:
With the development of modern psychology and the behavioral sciences, there is a more scientifically liberal (or liberally scientific) approach to human behavior. After studying a huge number of aberrant cases, modern psychology, taken together with sociology, history and evolutionary biology, has taught us not to simply condemn an action as 'evil', but to place a greater emphasis on recognizing humankind's innate propensity for violent behavior, which is explained by our evolutionary descent from amoral, competitively violent animals, and proved by mankind's long history of crimes, wars and conflicts. 
Thus many actions that would have been considered 'evil' in earlier centuries have now been reviewed and re-understood by socially liberal individuals and scientifically progressive fields of knowledge as either mentally ill, amoral (a word used by OP which implies a lack of moral awareness), callously selfish or at worst perversely immoral -- did something bad on purpose despite being well aware of the moral implications -- with several explanatory adjectives like callous, selfish, hateful and cruel: none of which is truly 'evil' in the classical or theological sense of the term because the darkest deeds of human beings can now be explained satisfactorily by either evolutionary biology or modern psychology. 
In short, whatever human action you might describe as 'evil' can be explained by somebody based on territorial expansionism & competition for resources; sex drive; psychological insecurity or outright insanity; innate biological aggression (read testosterone); or the very dangerous quest for grandeur that motivated Alexander, Napoleon and Adolph. What is truly evil? NOTHING, in the modern sense: it is just the human behaving like the innate animal. You might say

The lion is a cruel beast

but would you say

The lion is an evil beast?

At its worst, the human being is not humane, but is like the lion only.
In this context, many socially progressive individuals would avoid using the word 'evil' as an old-fashioned, outdated conceptual construction. Nor is there a need to use 'evil' when you have perfectly adequate words like amoral, perverse, hateful, callous, selfish and cruel!
Read: On Evil. Is there such a thing or force as evil? Or just bad deeds? -- from Psychology Today 

Note: the two links provided here (to articles in Wikipedia and Psychology Today websites) are only meant to reflect the overall philosophical approach to good and evil from religious and psychological perspectives; I have not used any material from either source to construct this answer, which is entirely my own written work.

