Crime's definition: an action or omission which constitutes an offence and is punishable by law.
If you live in a lawless society, then nothing can be defined as crime.
Is [a murder] justifiable?
justifiable: able to be shown to be right or reasonable; defensible.
Every action is justifiable, regardless of what consequence they bring. Just because a murder is justifiable from a subjective point of view doesn't mean people will give you a free pass.
Say if you kill someone, you are a murderer, but if there is a legit reason for the murdering then is it still a murder?
murder: the unlawful premeditated killing of one human being by another.
If you do a murder, you're a murderer. If you kill people, you're a killer lol.
A "legit" reason is subjective. It may affect the way a murderer is dealt with, but it doesn't change the definition of a murder.
Say you scam a gullible person and they agreed with your outrageous deal, then is it still a scam or just stupid people making stupid decision?
scam: a dishonest scheme; a fraud.
fraud: wrongful or criminal deception intended to result in financial or personal gain.
If you scam someone, it's a scam.
If you bring the outrageous deal while respecting all lawful requirements, it's technically not a scam.
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Though questions of crime and morality overlap, crimes are objective circumstances that relate to law. Morality is subjective and can vary for individuals, but laws are like written down morality. It's a convention that is imposed upon all citizens. (The content of laws may be subjective, but the fact that you follow them or not is objective)
Morals are subjective rules that a community agrees to follow. Failure to comply to your community morals will result in distrust, as you're likely to harm the community again. It is easier to get rid of a distrusted element, than mending a relationship/rebuilding trust.
That's why breaking morals and laws is more likely to result in your removal (banishment/exile/execution), than rehabilitation (needing to chaperone your ass to assess your trustworthiness).
Let's say you're my neighbor and you just murdered someone. Sure, you might never murder again and contribute greatly to the world, but what guarantees that you won't murder me if even law couldn't stop you from murdering before? It's not worth the hassle, so I'd rather you stay far from me.
murder in self-defense is murder. Murder during war is murder.
In my country, you're legally allowed to kill in self-defense if your life is threatened by illegal actions against you. Likewise, killing during war is technically legal since it's pushed by the government/law. Even if it results in death, legal actions cannot be technically be called murder. Maybe the term lawful homicide is used.