commit murder or commit a murder

[birdeen's call]

Which one would you say? (There is just one sample sentence in the poll. Comments on other possible sentences will be appreciated. There is no context, because I have just made the sentences up.)

 

[Atchan]

I have chosen the first option, but I cannot justify what let me to choose it :-D

I'm impatiently waiting the true one

 

[freezeframe]

There's no either, depending on context option.

 

[sunsunmoon]

Merriam-Webster's Learner's Dictionary
They were accused of conspiracy to commit murder.

 

[birdeen's call]

There's no either, depending on context option.

There's not. Is it possible to explain in what context you would say number 1 and in what context number 2?