I raised a similar question in another thread a few days ago. Genesis chapter 6 is interesting in that it reveals God's attitude toward humans, at least at that time. Gen.6:6,7: "And Jehovah felt regrets that he had made men in the earth, and he felt hurt at his heart. So Jehovah said: "I am going to wipe men whom I have created off the surface of the ground, from man to domestic animal, to moving animal and to flying creature of the heavens, because I do regret that I have made them."
I don't know for sure, but to me, it sounds like an emotional outburst, anger, hurt, regret, etc. The WT tries to temper what the word "regret" means, as if Jehovah didn't really mean it. But, the reality is, if it weren't for Noah "finding favor" in God's eyes, mankind would have been toast. And, yes you are right, all of this, if not most of it, because of what the angels had done in bringing violence to the earth. Where's the attributes of Jehovah in this (Love, Justice, Wisdom, Power). Alright, I suppose Power could stand alone without application of the other attributes, but in that case it becomes unreasoning Force.
If you want a really captivating slant on what happened, read 1 Enoch especially chapters 1-35. Enoch mentions details of what supposedly happened concerning these angels. A number of the books of Enoch were found among the Dead Sea scrolls, so it has some history. Jude alludes to the book of Enoch. This doesn't mean it's all true, some of it sounds like a fairy tale (kind of like Revelation).
I am left to wonder how much God's son may have influenced, even tempered God's anger toward humans at that time.
oldseeker