At a press conference Wednesday morning, Pennington County state's attorney Glenn Brenner addressed the September shooting of 21-year-old Dallas Two Bulls.
The city has completed its investigation, and no charges will be filed against Thomas Wilson.
Police say Wilson was in his house with Two Bulls' twin sister.
The sister called Two Bulls and apparently asked him to pick her up.
Police aren't exactly sure why, but Two Bulls kicked down Wilson's door when he got there and started to beat Wilson with a baseball bat.
Wilson got his gun and shot and killed Two Bulls.
Toxicology results show Two Bulls, his sister, and Wilson were all intoxicated at the time.
Brenner told reporters Wednesday morning the shooting was justifiable self-defense.
"We feel that under a reasonable person's standard, he was in genuine fear of his own life, and the force that he used to stop the assault was appropriate," Brenner said.
Police also found some of Wilson's property on Two Bulls, including a watch and other jewelry.