The bible doesn’t tell us a lot about the details of the antediluvian wickedness. Consorting with Neandertals could have been in the picture. Demon incarnations are commonly believed possible, which could further blur the genetic picture. We might not have been told about the wickedness because it might seem so infernally charming now.
Probably so. God doesn't tell us the details for a reason The meaning of 'Sons of God cavorting with the daughters of men' have been speculated over many times; the level of violence; the meaning of 'giants' (literal or figurative?), are left hazy for a reason.
We are curious lot, and we often speculate, but God tells us what we need to know - Salvation is through Jesus Christ alone.