The Nephilim (/ˈnɛfɪˌlɪm/; Hebrew: נְפִילִים Nəfīlīm) are mysterious beings or people in the Hebrew Bible who are described as being large and strong.
Indeed - but no text in scripture says that the term also means evil.
The Hebrew word Nephilim is sometimes translated as "giants"
agreed.
The fact that myth legend etc has evolved over time - does not change what we actually see in the text.
No Bible text suggests that sin is related to birth where some are born evil and others are born righteous.
Rather the Bible says all have a sinful nature and the Bible has no concept of "different species of humans" etc.
In addition Genesis 6 does not say "giants were born as a result of mixed marriages"
Nor does it say "humans born in mixed marriages need to be destroyed".
Rather it is because mankind became evil that they were destroyed.
5 Then the Lord saw that the wickedness of man
was great in the earth, and
that every intent of the thoughts of his heart
was only evil continually. 6 And the Lord was sorry that He had made man on the earth, and He was grieved in His heart. 7 So the Lord said, “I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them.” 8 But Noah found grace in the eyes of the Lord.
Does not say "wickedness of Nephilim was great" -- rather the entire focus is on 'man' -- "mankind" corrupt and wicked --
Not "physical defects" but rather moral corruption.