There were Biblical scholars from the early 1st century that have studied the ancient texts regarding angels and humans.
http://www.douglashamp.com/part-six-the-sons-of-god-according-to-ancient-sources/
Athenagoras
Church Father Athenagoras, AD 177 wrote in “Concerning the Angels and Giants†that it was the fallen angels who fathered the giants before the flood.
Just as with men, who have freedom of choice as to both virtue and vice […], so is it among the angels. Some, free agents, you will observe, such as they were created by God, continued in those things for which God had made and over which He had ordained them; but some outraged both the constitution of their nature and the government entrusted to them: namely, this ruler of matter and its various forms, and others of those who were placed about this first firmament […] these fell into impure love of virgins, and were subjugated by the flesh, and became negligent and wicked in the management of the things entrusted to him.†[ix]
Notice Athenagoras’ description of how these angels outraged (were not faithful) to the government that had been entrusted to them. This language of course corresponds to the language of 2 Peter 2 and Jude of the angels that did not keep their first estate (residence). Athenagoras further commented how these angels could no longer rise to where they once had been (heaven) and the souls of giants, who he says are in fact demons, wander the world.
These angels, then, who have fallen from heaven, and haunt the air and the earth, and are no longer able to rise to heavenly things, and the souls of the giants, which are the demons who wander about the world, perform actions similar, the one (that is, the demons) to the natures they have received, the other (that is, the angels) to the appetites they have indulged. [x]
Commodianus
Commodianus, A.D. 240, wrote how from angels’ seed the giants came about. Again, we see that the ancient Christian interpreter believed Genesis 6 to be referring to the comingling of angels and women which produced a hybrid race of giants, which confirms that when Jesus mentioned “as it was in the days of Noah, so will the coming of the Son of Man be†people would have thought about the Nephilim.
When Almighty God, to beautify the nature of the world, willed that earth should be visited by angels, when they were sent down they despised His laws. Such was the beauty of women, that it turned them aside; so that, being contaminated, they could not return to heaven. Rebels from God, they uttered words against Him. Then the Highest uttered His judgment against them; and from their seed giants are said to have been born. […] But the Almighty, because they were of an evil seed, did not approve that, when dead, they should be brought back from death. Whence wandering they now subvert many bodies, and it is such as these especially that ye this day worship and pray to as gods. [xi]
The Extant Writings of Julius Africanus
Julius Africanus (A.D. c. 160?- c. 240?) was the first to tentatively suggest that “sons of God†might be referring to the descendants of Seth and the “seed of men†could possibly be referring to descendants of Cain. However, he also conceded that it could just be angels as the text he was reading stated. Furthermore, it was by these angels that the race of giants was conceived. Augustine, however, was truly the first to state without a doubt that the sons of God simply meant sons of Seth.
When men multiplied on the earth, the angels of heaven came together with the daughters of men. In some copies I found “the sons of God.†What is meant by the Spirit, in my opinion, is that the descendants of Seth are called the sons of God on account of the righteous men and patriarchs who have sprung from him, even down to the Savior Himself; but that the descendants of Cain are named the seed of men as having nothing divine in them, on account of the wickedness of their race and the inequality of their nature, being a mixed people, and having stirred the indignation of God. But if it is thought that these refer to angels, we must take them to be those who deal with magic and jugglery, who taught the women the motions of the stars and the knowledge of things celestial, by whose power they conceived the giants as their children, by whom wickedness came to its height on the earth, until God decreed that the whole race of the living should perish in their impiety by the deluge.[xii]
Regarding your genocide question, canabalism and child sacrifice was practiced by the Canaanites would fall under "deplorable sinfulness".
Sure, drought may have worked, then again how would have Noah survived that? Unless he would have the capabilities to grow crops w/o water on his own, etc. A worldwide drought would have stunk of rotting flesh, no?