Potential spoilers ahead.
8MM could've been so much more, given that it was written by Andrew Walker who also wrote Seven, one of the most disturbing films I've seen. Unfortunately, we have Joel Schumacher at the helm which, given his recent tendency to go for style over substance (Batman and Robin), makes for a good-but-not-great thriller.
An eight mm reel of film depicting a scared young girl being killed by a masked man is the legacy of a wealthy industrialist. Private investigator Tom Welles (Nicholas Cage) is called upon by the widow to investigate whether this "snuff film" really is evidence of a crime or just a simulacrum. Welles is extremely disturbed by the film; as he tracks down the girl and her killers, he tries extremely hard to understand "why?", but to no avail.
While I think Nicholas Cage is a great actor, he has done better. Cage's acting improves as the movie progresses. I found his reaction to the snuff film utterly unconvincing. Likewise, the chemistry between him and his wife and daughter is non-existent. However, the portrayal of how he slips into the dark side is excellent, and is well-suited for his style of intense acting. The movie is peppered with interesting characters, including a couple sleazy pornographers, a sleazy lawyer, the young girl's mother, the industrialist's wife, and Welles' friend Max California (Joaquin Phoenix), but most of the screen time is devoted to Cage. The cinematography is pretty decent and complements the dark nature of the film well.
In 8MM, as with Seven, Walker appears to be capitalising on the notion that an ambiguous ending is generally bound to be a hit given the standard Hollywood fare where most things are nicely resolved at the end. In Walker's world, people appear to do the things they do for unfathomable reasons, but the reason "because we can" is a logical (and valid) extreme. In fact, it is so logical that Welles derives no satisfaction from the targets of his actions after he uncovers the truth. Schumacher, who has done great films like Flatliners, unfortunately depicts the ending in a positive light, as in A Time to Kill: vigilantism pays off. 8MM is definitely worth your money, but I personally think the movie would've been a lot better with an ambiguous ending like the one in Seven.
Amusingly enough, in the end, one of the killers taunts Welles, saying he kills because he wants to, because he likes to, and that he wasn't abused as a child or had any problems with his parents. But just before this occurs, we are shown that the killer was raised as a Faithful Christian, perhaps under his mother's strict yoke, which may well account for all the repression within him.