Since the murderer didn't know the victim, the detectives assumed the murder was a [...].
This is just an example, though, I'm not looking for a word for that kind of crime. Just a noun that means "something that is randomly chosen".
Something chosen at random can be called arbitrary.
arbitrary (adj.) - Based on random choice or personal whim, rather than any reason or system
I can think of
indiscriminately - In a random manner; unsystematically:
or maybe even a less profound alternative like unselective or aimless, depending on the context
I think indiscriminately might be a better choice than arbitrary, because I believe the latter has a connotation of personal choice based on a mood. I am, however, not too sure about this and might be corrected accordingly.
Randomness is the noun form of random, but it doesn't work for usage as "such and such was a randomness".
You can use it in the phrase act of randomness, which may be what you're looking for. Unfortunately I think "of randomness" is essentially just an adjective, and therefore it might not be any better than the many synonyms of random that have been given as answers.
a chance occurrence
a random act
a crime of opportunity
a shot in the the dark: a random conjecture
throw of the dice: a risky attempt to do or achieve something
If you want a noun for "something that is randomly chosen", you could use the noun form of chosen - random/arbitrary choice. Or to be precise, in probability, the result of a random experiment is called an outcome.
I think accidental can convey the 'random' aspect you are referring to:
- Happening by chance, unintentionally, or unexpectedly. (ODO)
How about caprice:
a sudden, impulsive, and seemingly unmotivated notion or action
In the context of murder, a good adjective is senseless. A senseless murder is one that occurred for no particular explanatory reason. For example, if a mugger were to demand someone's wallet and the victim fought back, we could understand why he might shoot the victim. But if he shoots the victim after receiving the wallet just because he can, then that is senseless.
See senseless (meaning c)
: done or happening for no reason ; destitute of, deficient in, or contrary to sense: as
a : unconscious
b : foolish, stupid
c : meaningless
A strong word, generally left to the realm of science : "stochasticity".
Since the murderer didn't know the victim, the detectives assumed the murder was a stochasticity.
- the quality of lacking any predictable order or plan
- Synonyms:haphazardness, noise, randomness
The term stochastic occurs in a wide variety of professional or academic fields to describe events or systems that are unpredictable due to the influence of a random variable. The word "stochastic" comes from the Greek word στόχος (stokhos, "aim"). (wiki)
Items (but not usually people) are sometimes referred to as sundries if they are haphazard, miscellaneous, or otherwise not important enough to refer to by name. As per the Oxford dictionaries:
sundries noun (plural sundries)
Various items not important enough to be mentioned individually: "a drugstore selling magazines, newspapers, and sundries".
[ATTRIBUTIVE] Of various kinds; several: "prawn and garlic vol-au-vents and sundry other delicacies".
Opportunistic describes crimes committed without a specific target, based on opportunities which arise, rather than being individually planned. In your case this would suggest that to commit murder was the primary goal of the crime, as opposed to a "robbery gone wrong".
In the case of murder, it's not often going to be the case that a murder is "opportunistic" unless the criminal is very depraved indeed. I believe most murders either target a particular individual the murderer have a motive for killing, or are unplanned consequences of other crimes such as burglary or robbery or kidnapping which have "gone wrong".
Is "a blind murder" fitting your need?
2. Lacking perception, awareness, or judgement:
2.1 Not controlled by reason:
they left in blind panic
- in a jumble
- a muddle
I admit it's not the perfect word for what you are describing, but it's really fun to say.
something that happens by chance http://www.merriam-webster.com/dictionary/happenstance
a situation in which events happen at the same time in a way that is not planned or expected http://www.merriam-webster.com/dictionary/coincidence
Freak occurrence (vocabulary.com)
A freak occurrence, on the other hand, is not necessarily a bad thing, but connotes something irregular or unexpected.
It could have been someone else, somewhere else. The James Bulger murder was a one-off, and nothing like that will happen again for a long, long time. The Damilola Taylor murder was not. It could happen again next week, next month.
My suggestion doesn't fit neither the OP's title nor the question in the body, a one-off is not random by any stretch of the imagination, and it is not chosen, but it is a noun which expresses a unique, never-to-be-repeated event or thing.