A Clockwork Orange

Near future England:  Teenage gang leader Alex (played by Malcolm McDowell) leads his three friends (or "droogs") on nightly rampages of theft and savage violence against whoever is unlucky enough to encounter them.  Tiring of Alex's arrogance, his friends set him up to be arrested after their latest attack goes fatally wrong.  Alex is convicted and sentenced to fourteen years in prison.  After two years inside, Alex is submitted to an experimental treatment called the "Ludovico Technique" which is intended to cure criminality by making the subject unable to act violently.  Alex is released after the treatment and soon finds that where he was once the predator, he is now the prey.

Released in 1971, written and directed by Stanley Kubrick, and based on the 1962 novel by Anthony Burgess, this is hugely controversial film is now acclaimed as a modern classic.  It is still a shocking film, in fact I would say that it would probably not get made today. because it would just be too problematic.  Not necessarily because of the on-screen violence, which is heavily stylised and more shadowplay than graphic blood and gore, but because we are invited to like and sympathise with a  brutal, unrepentant rapist and murderer.  The entire film is shown through Alex's eyes, and he breaks the fourth wall with his voice-over narration (addressing the audience directly as "my brothers and only friends").  The first part of the film, depicting Alex's crimes is heavily stylised, whereas the latter part of the film, where Alex becomes the victim, the violence is much more realistically depicted thereby inviting the audience to enjoy Alex's rampages at a distance, but to sympathise with his own sufferings.  Which is, of course, how Alex would see it.  Also there is the towering performance of Malcolm McDowell as Alex, alternately threatening and innocent, it is a career best performance, and he is in pretty much every scene of the film. 
The film is relatively faithful to the novel, although it discards the final chapter of the original, British version of the book.  There are some odd elements in regards to the book, in which Alex's age is stated as being fourteen.  Malcolm McDowell was in his late twenties when he made the film, and yet he is constantly referred to as a child, despite being clearly an adult.
The film depicts a very seventies future, and it really is a product of it's time and place, it feels more like an alternate early seventies than a futuristic piece.  The novel was written in an invented slang called "Nadsat" and this is memorably retained in the film, although toned down.  Burgess invented nadsat because he felt that if he wrote it in then-current slang then novel would feel dated as soon as it was published, and he was right.
The film was famously withdrawn from release in Britain by Kubrick himself and was not legally available there until after his death.
It is still a troubling, shocking and powerful experience. 

Sometimes They Come Back

In 1990, high school history teacher Jim Norman (Tim Matheson) returns to his childhood home town for the first time in 27 years, with his wife Sally (Brooke Adams) and young son Scott (Robert Hy Gorman).
In 1963, nine year old Jim (Zachary Ball) and his fifteen year old brother Wayne (Chris Demetral) are walking through a railway tunnel on the way to the library when they are set upon by a gang of teenage greasers who kill Wayne, but are themselves almost immediately killed by a train.
Despite still being haunted by nightmares of his brother's death, Jim and his family settle in, and he starts teaching at the local high school, bonding with some students and making enemies of others.  However, the students who Jim gets close to start dying in apparent suicides, and are replaced in class by students who look suspiciously like the gang who murdered Wayne.
Jim realises that the spirits of the dead gang members have returned and are set on revenge.

This film was originally made for TV and was first broadcast in 1991, directed by Tom McLoughlin.  Based on a short story by Stephen King, it's a pretty by the numbers horror film, that features plenty of King tropes.  Made on a low budget without particular style or flair, with a cast of solid performers, including veteran William Sanderson, who all try their best with stodgy material (pity Brooke Adams, who really has nothing to do here at all except be alternately supportive and scared).  The story is not one of Stephen King's best and the screenwriters, Lawrence Konner and Mark Rosenthal, have trouble punching it up to feature length.  The film is at it's best when the focus is on the ghoulish greasers who (as played by Robert Rusler, Bentley Mitchum and Nicholas Sadler) are ominous and threatening, although we never find out enough about them, they have no backgrounds or personalities, and are portrayed as pack animals (they growl, bray and howl like wolves).
Probably due to the limits of television in 1991, the film is light on blood and gore.  At times it seems more like a supernatural drama about coming to terms with grief, than it is about shocks and scares, and the two sides don't gel together.  The biggest problem that the film has is that it offers nothing new.  Really, if you're interested at all in horror, than you've seen it all before.