In real life, the reasons for murder vary widely; however, in fiction and in the most talked-about murders, it seems to me they fall into four categories, all of which may overlap in some cases:

English murders. In his essay "Decline of the English Murder" George Orwell described the "perfect murder" as follows:

The murderer should be a man of the professional class--a dentist or a solicitor, say--living an intensely respectable life somewhere in the suburbs . . . He should be either chairman of the local Conservative Party branch, or a leading Nonconformist and strong Temperance advocate. He should go astray through cherishing a guilty passion . . . and should only bring himself to the point of murder after long wrestles with his conscience . . . he should commit murder because this seems to him less disgraceful, and less damaging to his career, than being detected in adultery.
That pretty much says it all. One example is Dr. Crippen.

Bad-man murders. I'm referring to anyone driven to murder by the sheer suckiness of their life. They tend to tug at the ol' heartstrings for various reasons. A lot of American "badman ballads" are like this, where typical excuses include gambling, cheating lovers and "just to watch him die" (Johnny Cash, Folsom Prison Blues). These songs tend to focus on going to jail and hanging as much as the actual murder. Here's one example from "Ain't Nobody's Business" by Mississippi John Hurt:

Some old morning gonna wake up crazy,
Gonna grab my gun, gonna kill my baby.
And "John Hardy," supposedly written by John Hardy himself:
John Hardy was a desperate little man.
Carried two guns every day,
He shot a man on the West Virginia line,
You should have seen John Hardy getting away, poor boy.
The DC snipers are also in this category; Lee Boyd Malvo is the quintessential troubled kid, having been abandoned by his father.

Politically motivated murders. Self-explanatory, but usually the only way these become popular is when they overlap with the previous category and people can sympathize with the goals of the murderer. As a general rule, murders don't make their way into fiction or a nation's long-term memory unless people can sympathize with the victim, murderer, or both. The Oklahoma City bombing, for example, won't stir our emotions twenty years from now unless we happen to have extreme libertarian views like Timothy McVeigh. (I won't consider Islamic terrorists here, since they're part of an organization, and soldiers are a different matter altogether. I only know a little about Islamic culture, but nothing I know indicates they take pleasure in reading about solitary criminals.) A very good example is the traditional folk song "Jesse James":

Jesse James was a lad who killed many a man,
He robbed the Glendale train,
He stole from the rich and he gave to the poor,
He'd a hand and a heart and a brain.
Poor Jesse had a wife who mourned for his life . . .
This category would also include Columbine and other school shootings. It would take pages to count the things they were rebelling against; generally it was everything they associated with the social structure of high school. I don't know about you, but when I saw those jocks on TV tearfully explaining that the shooters were jealous of their superior social and athletic prowess, and that preacher blaming video games, rock music and atheism but saying guns are a "side issue," my immediate reaction was "fuck you, asshole." Michael Moore pointed out a document released by some government folks about signs that your child could be a school shooter: among them things like "rebels against authority" and "difficulty fitting in" and of course, drug use. You'd think school homicides were as widespread as, say, clinical depression. For that matter I wonder how many parents of suicidally depressed teenagers were worried about them becoming another school shooter. Badman ballads and school shootings are different from English murders in that you don't feel much sympathy for the victims, certainly not that girl who professed her belief in God even though she knew full well she'd get shot for it. If there's a God I bet he laughed at her for being so stupid.

Sick-ass motherfucker murders. The most delightful of the three, and also the most American. These are committed for no particular reason except to become attractive to the opposite sex ("Oh baby! Being a homicidal maniac is so nasty!"). Emotionally, the pleasure one derives from the sexual aspect of it is indistinguishable from the pleasure derived from the violence. Even when there's no actual sex involved, these murders seem to be about the sheer pleasure and adrenaline rush of violence. Gangsta rap makes considerable use of this type of murder. For example, from the song "Straight Outta Compton" by N.W.A.:

Shoot a motherfucker in a minute.
I find a good piece of pussy; I go up in it.
Thematically as well as musically, rap is influenced by the blues, and yet they are clearly very different. Unlike in blues and folk murder ballads, rappers seldom sing about getting caught or going to jail. This kind of murder goes all the way back to American pulp fiction. Take for example Robert Bloch's brilliant short story "A Toy for Juliet" from Harlan Ellison's anthology Dangerous Visions. That story gave me quite a hard-on. It's about a girl in the future whose grandfather brings her people from the past so she can have sex with them and then kill them. In the introduction to the story, Bloch says the story's purpose is an "examination of violence in our society." Right Mr. Bloch, it's my duty as a scholar to read this story because it's so artistically profound, *wink* I understand. In another damn good essay by Orwell, "Raffles and Miss Blandish," he analyzes fiction that exploits sex and violence to produce thrills in the reader. He points out that although the English are increasingly enjoying such fiction, mostly only Americans are very skilled at crafting it. One more reason to be proud to be American. Also, in "Decline of the English Murder," he examines the emotional implications of the Cleft Chin Murder, in which an eighteen-year-old girl meets an American army deserter. She falsely claims to be a strip-tease artist, and he falsely claims to be a Chicago gangster, and they proceed to kill a few people just to feel tough. Orwell says: "The background was not domesticity, but the anonymous life of the dance-halls and the false values of the American film." Such values are dangerous, says Orwell in "Miss Blandish," calling them "a daydream appropriate to a totalitarian age" because, even if they don't directly advocate any political beliefs, they promote the fascist idea that the strongest person is always morally right, and Orwell never uses the word "fascist" lightly. Orwell's observation is especially evident in today's gangsta rap. From "Fuck tha Police" by NWA:
Without a gun and a badge what do ya got?
A sucker in a uniform waiting to get shot.