How might the killing of Edward be seen as self defense?

While Mr. Loomis is talking in his sleep (delerium), Ann comes to the conclusion that Mr. Loomis and Edward had a disagreement. She realizes that Mr. Loomis and Edward had been locked in their lab together. Edward wanted to use the radiation suit to go and check on his family, but Mr. Loomis did not want him to. Ann believes that Mr. Loomis shot Edward when he tried to leave with the suit. Several patched holes in the suit seem to support this conclusion.