Was Noah's flood because He was mad at His people or something else?
Well, I guess I'd have to answer a bit of both. First, it is clear that God was grieved with His people. Gen. 6:5-7 states God's case against His people in pretty strong terms. He was sorry He made us because the only thing we thought about was doing evil. That's a paraphrase, but it is the gist of the story.
Yet something else was at work as well. The angels that fell with Lucifer had gone in unto the women in an effort to keep there from being a pure line of humanity from Adam and Eve to Jesus that was not contaminated with their genes. The Bible is clear that their offspring formed some of the giant races of which both the Bible and legends of the people surrounding Israel speak.
Noah found grace in the eyes of the Lord (Gen. 6:8). It is important to see what the Bible says the reason for this is. It is not, as is commonly supposed, that He was a perfect man although the Bible does say God found him to be just. What the Bible says is that Noah was perfect in his generations. His genetic history hadn't been messed up yet by the fallen angels, and obviously his wife's genes were not messed up either. So God picked that particular family to maintain and save out of all those around them because they were just and had a clear genetic line to Adam.
There is a lot of debate over Noah's flood. I will state my opinion, and reiterate that this is only opinion and could well be wrong. It is my belief that Noah's flood was universal - as far as Noah could determine. The flood of Noah was the result of the Mediterranean flowing into the Black Sea basin the first time when the Bosporus strait opened. The Black Sea is very large, and if Noah lived in the center of the basin, when the flood came he would have been lifted up in the ark and may well not have been able to see any land or any shoreline around him. For all he could tell, the world was flooded.
As he moved to shore and came to rest, it could well appear that the waters were receding as he would begin to see the surrounding mountains that he may have never before seen or noticed. At any rate, if it was a localized flood, he would not have had to save every animal in the world but only those in the surrounding area.
I also believe that it is possible that Noah was living to the East of that range of mountains, and the filling of the Black Sea could have precipitated enough rain in the region to cause a flood on the eastern side which subsided after due time. The only problem with him being on the west side and all the people drowning by that filling is that the Black Sea is obviously still there, so if you are a purist who believes the water went away rather than the ark just reaching a shore, then this is a solution for you. It also has the merit that a wind could have blown the rain north and caused the water source to slow to normal allowing the flood waters to abate on their normal course.
I will accept without question that God could have arranged the flood to be truly universal and miraculously gotten all the animals there, but the flooding of the Black Sea historically took place around the time of Noah and it seems to make the most sense to me regardless of whether he was on the west or east of the mountain range. You are welcome to your own beliefs on the matter.
What can we learn about God's nature from this story. Well - first He is holy. He is also loving and patient and waited till the very last of mankind in that area remained untouched by the work of Lucifer hoping that the dwindling numbers would turn to Him instead of going with the seemingly more powerful giant races. When it was down to the wire, He acted with a backup plan that eliminated that particular threat of Lucifer completely.
He was clear about telling Noah what was going to happen, just as He has been clear about warning mankind of what will happen during the lesser and greater tribulation if you don't go in the rapture or what will happen to you eternally if you don't accept Christ as Savior. Because Noah obeyed the Lord, he and his family were saved. If you haven't accepted Christ yet, you should - while there is still time.