I do have a reason for asking which I shall reveal later.

What, exactly, do you understand, in literary terms, the word "hero" to mean?

In a Classical context, a hero was the son of a god(dess) and a human, e.g., Heracles (or Hercules). In literary terms, a hero (or protagonist) is the character whom the story is about.

I consider it to mean principal male character of the story, though he should be good.

To me, in literature in general, a hero is someone who overcomes adversity with a display of courage. It can also be used, more loosely, to mean the protagonist. In ancient myths and legends, the hero is a demi-god, as zmj mentioned.

Let me tell you why I ask.

The Guardian is printing a list of "1000 novels everyone must read". In the crime section one of them is Bret Easton Ellis's Amercan Psycho. In the decription it says that the "hero" nails an ex-girlfriend's hands to the floor, cuts out her tongue, forces her to fellate him and kills her and that he also axes a gay man he meets on the street and casually eviscerates his dog.

I was wondering how the word "hero" could possibly be justified in this particular instance.

There is a bit more about this on my blog.

I think "hero" is misused in that context and many others. The lead character is the "protagonist" in that particular book, with nothing heroic about him.

The problem is "hero" is one of those words that has too many definitions, to many different qualificatins which make it difficult, if not impossible, to determine exactly where a particular person would fit.

A hero can risk his/her life for a lofty goal;
can be someone whose life is worth admiring and emulating; or can be someone who performs a task at an extraordinary level, as in sports.

The differences in achievement make using the same term for each ludicrous. Should we call a soldier whose exploits put his/her life in danger by the same word -- hero -- as a ballplayer (earning millions of dollars to hit a ball) who wins a sporting event?

I agree; the correct word is "protagonist", not "hero".

the term anti-hero has been used in this context, but is still usually someone who is admired in spite of (or due to?) his weaknesses. this hardly applies here either.

aside: "cuts out her tongue, forces her to fellate him.. also axes a gay man he meets on the street and casually eviscerates his dog" ... surely not in that order! (sorry)

I completely agree that the word "hero" is incorrectly use there.

According to at least one source (Wordweb) a meaning of hero is "The principal character in a play or movie or novel or poem".

In that sense this unpleasnat character is a hero - if in no other.

Of course, the word has been debased and we now refer to the heroic deeds of those who are simply doing their jobs. The footballer who scores a winning goal is freqently referred to as "the hero of the match" by the red-tops - when in truth he is not more of a hero than am I for getting up at 0530 to catch the 0620 'bus to make sure that my customers get their training on time. Consistently doing a good job is worthy of praise - but hardly heroic.

Another easy one.
Firemen are heros.
Everyone else is a bum.

It's a corruption of gyros, and is a sandwich.