Another question with this topic.... my lawyer student asked me if extortion means for example if someone �knows someone �s secret � and asks that person money in order not to reveal what s/he knows...otherwise, s/he �ll tell the others the truth... Are we in the correct way??? Thank you so much in advance!!! Regards!!!  

 

Killer is not a legal term. It can actually be used both as a noun and as an adjective. When used as a noun, it can mean either someone or something that kills, or someone or something (such as an animal) that enjoys killing. It is not exactly the same as hitman/hitwoman, which refer to someone who is paid to kill a specific person. Killer as an adjective can refer to a killer whale (orca), or to something that is very difficult (Wow! Killer test today!) or really nice (Killer shoes! Where did you get those?) The latter two uses are slang; killer whale is standard.

 

A murder is the deliberate and unlawful killing of another person. Manslaughter is the killing of another person through negligence. There �s also self-defense, accidental death, and killing as an act of war. A murderer is a person who is convicted of illegally killing another person through deliberate actions. By extension, some people will call a person who wrongfully causes the death of another living being as a murderer. Many Americans called the American football player Michael Vick a murderer after he was convicted of dog fighting. He never killed a person, but he was responsible for the deaths of several dogs in the dog fighting.

 

Your lawyer friend is correct; extortion is the unlawful use of force or threat to gain property or money. The specific kind of extortion your student mentioned is usually called blackmail.

 

Hope this helps!

For a definitive answer, I would ask a law library or lawyer. 
The resources I found online didn �t seem conclusive to me.

They all had definitions for  the noun �murder � but no entry for who commits the crime.