Presumably he didn’t believe the kidnapper would receive a proportionate sentence for his crime and was ready to serve time himself.

From Brave AI:

“The man who shot his son's kidnapper in broad daylight in 1984 was Gary Plauche. He shot and killed Jeffrey Doucet, the man who had kidnapped and molested his 11-year-old son, Jody.

Gary Plauche was given a 7-year suspended sentence and received no prison time for his actions.”