It is very common for serial killers to experiment with killing animals before they move on to humans, or kill animals when they do not have access to humans. Killing animals is actually part of the "homicidal triad" of behaviors that are usually present in children who become serial killers. The three features of the homicidal triad are bedwetting, fire-starting and cruelty to animals as a child.
Harvey’s dysfunctional childhood definitely contributed to his development into a serial killer. He was abandoned and abused. This does not excuse his behavior, but does make it more likely that he exhibited all three serial killer attributes of the homicidal triad.
He had killed animals, taking lesser lives to keep from killing a child. (p. 127)
It is not completely uncommon for serial killers to use animals as surrogates, especially in the beginning. Harvey does seem to have felt remorse for killing at first, which is why he used animals. Eventually, he abandoned all efforts to stop killing because he could no longer get the release he needed from killing animals, and had to move on to raping and murdering young girls.