Tedd Reuther claims his son, daughter-in-law and granddaughter were being chased by a neighbor's pit bull last week.
When he went outside to see what was going on, Reuther said the dog was "growling; teeth were out, ears were back, tail was not wagging."
He said the dog launched at him, forcing him to fire his rifle.
"It was either that or die," said Reuther.
Police said officers at the scene determined the killing of the dog was done in self-defense.
The pit bull's owner, Sherri Adams, was devastated and wished her dog had not been killed.
"[They] could've done 50 things. They could've come out with a broomstick. My dog would've been afraid of the broom," said Adams.
"I looked back out and noticed the note waving in the wind a little; it caught my attention," Reuther said.
The note read: "Your [sic] a liar and a murderer. You deserve to be in jail you son of a b----."
"Now they're targeting me for defending myself and my family," said Tedd Reuther. "They didn't see any of this. They don't know what's going on. These people don't know me."
Police recommend anyone who is confronted by a vicious dog to find a place of safety and call 911.
However, if you can't bring all your livestock, neighbors and pets inside, get a gun and shoot it.