Crazy, nuts, wacko, freak, cuckoo, kook
People often ask what makes someone "crazy," but "crazy" colloquially tends to mean "without explanation," and there's always a reason for disturbed behavior; hence, nobody is really "crazy."
Experts disagree on how inappropriate these terms are. Some say that calling behavior, decisions, or ideas crazy, even jokingly, is hurtful to those whom society might, in its ignorance, call crazy, too. Others say that political correctness can only go so far and that as long as the term isn't being used to label someone that it's not a problem.
According to the December 8, 2006 NAMI StigmaBusters 
alert, "The 'Obsessive Compulsive Action Figure' led to healthy dialogue among StigmaBusters about the uses of humor and whether mental illness is ever funny. An informal survey of readers revealed the following opinions:"
50% — Yes, but only when it's not stigmatizing in nature; i.e., making fun of the illness, not the person, and especially if it educates others or helps a person cope.
25% — Yes, but only when we are laughing among ourselves, consumers, families and therapists.
14% — Never
11% — Yes. Lighten up. Laughter is the best medicine.
A lot of people come into therapy worried that being there makes them "crazy;" from the perspective of the therapist, seeking help for a problem is one of the least "crazy" things someone can do. Since therapists have been trained to understand why people do the things they do, behaviors, even odd ones, usually make plenty of sense in the context of someone's life.
Writing Suggestions
As tempting as it may be, try to keep your therapist's use of words like "crazy" or "freak" to a minimum. I've heard people use it jokingly outside of work (Your friend decides that shaving his head is a good idea; you say, "You are such a freak") or even in reference to non-pathological behavior ("What? You didn't like that movie? You're crazy!") but no matter how sick someone is, we don't really see them as "crazy."
When the Bizarre is Kind of Normal
It's also easy to accidentally label a pretty standard pathological behavior as extraordinarily bizarre. In the movie The Terminator, Dr. Silberman, the criminal psychologist asked to assess the time-traveling hero, isn't so bright. He's played off as a kind of comic relief in the films, because he's always so clueless, something he demonstrates nicely when he acts like Reece's behavior is unusually bizarre when it isn't at all. The script partial is long, so it's on a separate page.
If we pretend for a minute that Reece isn't 100% right about the Terminator and everything else he's saying, he sounds like he has a paranoid delusion, which is exactly what Silberman says. And while Reece's delusions are certainly bizarre, they remind me of Capgras Syndrome (the belief that relatives have been exchanged for robots that look exactly like them) or Intermetamorphosis (the belief that people are trading identities, even though they look the same). As weird as those sound, they're common enough to have names!
Monstrous Behavior
Even people who commit the most atrocious crimes do so for some reason. The reason doesn't make the behavior okay or reduce the appropriateness of punishment, but it does mean that therapists see these people as people with serious problems rather than as monsters. Pedophilia, murder, rape, torture, theft, and mutilation are never okay, and it's much easier to call people who commit these crimes monsters than to recognize that they are very much like each of us, but the world isn't black and white, and nobody knows that better than psychological professionals.
In novels, movies, and TV, psychological professionals' exclamations that someone is a monster (etc., etc.) are often meant to emphasize that the villain is unusually disturbed and therefore can be killed without remorse. In the context of horror movies, where this approach is particularly common, the point of the story isn't really how the killer got that way; it's that he's on the rampage and the characters in the story could be next.
The explanations on the genesis of horror films' villains can get creative. For example, lots of people have remarked that unless something pretty extraordinary has happened in human biology, Freddy Kruger can't be the "son of 100 maniacs." Though I suppose you could argue that Amanda Kruger's state after the attack could affect the development of the child (never mind the maniac part, the violence was enough).
Antisocial Personality Disorder
Or you could take an easier route and just have the child raised in the kind of environment that does create antisocial personalities; at the time most of these movies were made, we didn't understand antisocial personality well enough for the writers to have used that explanation.
Some examples of villains who are "beyond help," all taken verbatim from the films' scripts. Of these, American Psycho is the most serious, and Bateman's voiceover captures the feel of antisocial personality well.
Halloween (script) 
SAM LOOMIS [described in the script as a clinical psychiatrist]: I met [Michael Myers] 15 years ago. I was told there was nothing left. No reason, no conscience, no understanding, not even the most rudimentary sense of life or death, of good or evil, right or wrong. I met this 6-year-old child with this blank, pale, emotionless face and the blackest eyes. The devil's eyes. I spent 8 years trying to reach him, and then another seven trying to keep him locked up because I realized that what was living behind that boys eyes was purely and simply...evil.
Nightmare on Elm Street 3: Dream Warriors (script)
NUN This is where it began.
NEIL This wing's been closed for years. (looking around) What was this place?
NUN Purgatory...fashioned by the hands of men. Twisted, lost souls, the worst of the criminally insane, were locked away in here like animals.
NEIL The whole facility was shut down in the Forties, wasn't it? Some kind of scandal...
NUN (she nods) A young girl on the staff was accidentally locked in here over the holidays. The inmates managed to keep her hidden for days. She was raped...hundreds of times. When they found her, she was barely alive...and with child.
NEIL (softly) My God.
NUN That girl was Amanda Krueger. Her child --
NEIL Freddy.
NUN The bastard son of a hundred maniacs.
Friday the 13th (script)
CRAZY RALPH: I'm the Messenger of God. You're doomed if you stay here. This place is cursed, cursed. It's got a death curse.
ALICE: Who are you?
MARCIE: What do you want?
CRAZY RALPH: God sent me.
NED: Get out of here, man.
CRAZY RALPH: I've gotta warn ya... you're doomed if you stay. Go, go (he walks out).
NED: I think we just met Ralph.
ALICE: God, what's next?
American Psycho (script)
BATEMAN (V.O.) There are no more barriers to cross. All I have in common with the uncontrollable and the insane, the vicious and the evil, all the mayhem I have caused and my utter indifference toward it, I have now surpassed...
INT. BATEMAN'S OFFICE - DAY Jean is alone in Bateman's office, looking through his diary. We see the pages that she is looking at. They are filled with doodles of mutilated women and their names...Jean looks lost and frightened, and begins to cry.
BATEMAN (V.O.) My pain is constant and sharp and I do not hope for a better world for anyone. I fact I want my pain to be inflicted on others. I want no escape. But even after admitting this, there is no catharsis. I gain no deeper knowledge about myself, no new knowledge can be extracted from my telling. There has been no reason for me to tell you any of this. This confession has meant nothing...