A few billion??? I think your math is a bit off.
And how would it occur to Cain that the offspring of a brother he didn’t yet have would pose a threat to him?
You are at complete liberty to provide your own guesstimates on numbers if you care. The point that is relevant here is that there are going to be a lot of people around to develop long term family feuds. Cain is revealing his conscience that he knows those who are in current existence would want to kill him (implicit that there are other brothers and sisters - not to mention Adam’s rage). And unlike us in our current degraded moral decline Cain was not a complete idiot and knew where babies came from since he was one at one time surmised that mom and dad would be making more and could do the math that there would be enough family around to hold a grudge for as long as humans lived.
Borrowing from another external source let me put it this way:
According to the simple math, if Cain waited to marry until he was about 200 years old, he probably had several women to choose from, providing some migrated eastward to Nod with other family members. If he waited another 200 years to build a city, he could have had at least a few thousand people to help him, again assuming some migration occurred. It is not entirely impossible that Cain had sisters from whom to take a wife even before his banishment and the birth of Seth. The text does not tell us. Assuming only a 50% active fertility cycle and no major wars/disease we have:
Here are the numbers:
year_________reproducing couples________children born_________ total population
0| 1| 0| 2
50| 1| 0| 2
100| 1| 10| 12
150| 6| 30| 42
200| 21| 100| 142
250| 71| 352| 494
300| 247| 1210| 1704
350| 852| 4180| 5884
400| 2941| 14,450| 20,334
450| 10,167| 49,892| 70,226
500| 35,113| 172,358| 242,584
550| 121,292| 595,378| 837,962
600| 418,980| 2,056,530| 2,894,492
650| 1,447,245| 7,103,862| 9,998,364
700| 4,999,176| 24,538,536| 34,536,930
750| 17,268,444| 84,762,338| 119,299,368
800| 59,649,613| 292,790,780| 412,090,500
850| 206,045,003| 1,011,374,120| 1,423,465,830
900 711,732,063| 3,493,544,650| 4,917,014,660
And your point in all this is what - to discredit me?
James