In your example (死は今ではない先のもの) the meaning would be 'Death is something ahead of you', i.e., in the future.
If you think of 先 as something 'ahead of you' it would mean the future. For example: 先の事を考える 'Think about the future'. 'Before you' and 'Ahead of you' are the same.
Something can be before me, which means that it is also ‘ahead’ of me. On the other hand, something can come before me, in which case it is ‘behind’ me (chronologically).
先 has many meanings. The proper one can be gleaned from the context surrounding it, according to this Q&A response:
Also, here's a useful explanation from kanjibunka.com here.