The Hebrew word ben when it stands alone in a sentence means "son", usually in the sense of parent and child, for example in Genesis 4:25 (NIV)
And Adam knew his wife again; and she bare a son
When ben is used in an adjectival construct, it does not mean "son" at all, it means
- result of or derived from
- subject to
- being a member of a particular class
One interesting exception is Genesis 35:18 (NIV):
As she breathed her last—for she was dying—she named her son Ben-Oni
Where both usages are intended, "son", and "result of my pain" and this gives the verse its poignancy in the original. Note that the MT Hebrew actually says "she named him ben oni". The "her son" of the NIV is an interpolation.
Apart from this exception, in the general case for example, when a person is described as being ben hamesh meot shana as said of Noah in Genesis 5:32, it means that Noah was in a class of things that are 500 years old, that is Noah was 500 years old. It does not mean that "Noah was a son of 500 years". That type of translation makes no sense.
In I Samuel 30:31, ben is used in an adjectival construct ben-mavet, meaning subject to death, or in the class of things that are dead. In other words, Saul is saying that David has a death warrant.
The same usage applies to the Hebrew bat, daughter, and the same usage applies to the Aramaic cognates bar and bara.