Freeman's character declares the killer to be "preaching a sermon." So he is. Later, when they finally catch him, he makes the sermon explicit. He points out that all think him an evil monster, but he contends that he is no worse a sinner than those he killed or anybody else. The lives he ended were pathetic ones---a glutton so repulsive that one could not eat while looking at him without vomiting, a woman who was so ugly on the inside that she cared only how she looked on the outside, etc. The killer contended that we have lost sight of the horror of these sins (and the others) because they are so common. He intended his acts to highlight the horror of these "deadly" sins.
The real horror of this film is that everyone leaves the theater thinking (no, it goes deeper than thought) "he had a point." So he did. (Although the Bible would certainly not condone the methodology of his sermon, it agrees with the point. Read Romans 3:23 and 6:23).
©C. David Hess