When people express surprise at how awful people can be, I express surprise at their ignorance of total depravity. People. Are. Essentially. Evil., Not. Basically. Good.
Our real sense of surprise, then, should erupt not when people do awful things, but when they do good things.
If you doubt it, you simply don’t know anything about Scripture’s anthropology:
There is none righteous, no, not one; There is none who understands; There is none who seeks after God. They have all turned aside; They have together become unprofitable; There is none who does good, no, not one. Their throat is an open tomb; With their tongues they have practiced deceit”; “The poison of asps is under their lips. Whose mouth is full of cursing and bitterness. Their feet are swift to shed blood; Destruction and misery are in their ways; And the way of peace they have not known. There is no fear of God before their eyes. (Rom. 3:10-18)
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. And the LORD was sorry that He had made man on the earth, and He was grieved in His heart. (Gen. 6:5-6)
That’s humankind in its essence and being. Good is foreign to humanity and is a surprise when it is found.