Whoever the sons of God were, the point going into verse 6 stays the same: violence and sin had spread so far that God was grieved. That’s Genesis 6:6, the verse about God regretting He made humanity. He was sorrowful over human wickedness and was going to respond with judgment.
On 6:1-4, Christians have read the ‘sons of God’ either as heavenly beings or as humans (Seth’s descendants or powerful rulers who took the ‘daughters of men’ for themselves). The passage doesn’t say the Nephilim were giants born from those unions, that they were around in that era and were famous warriors.