The story begins in a liquor store and Gerald (George Nader) goes inside to buy a carton of beer. But it turns out that there's an armed robber there. After brandishing the gun and threatening him and the woman at the counter, the guy leaves with the money. The lady hands Gerald her gun and he then chases the young man...shooting him down as he tried to make his getaway. Later, it turns out that the teen's gun was empty...and Gerald is obviously affected by this and the boy's death.
"Self Defense" is an extremely powerful episode. You learn that Gerald was affected by the incident more than most, as he'd been in WWII and obviously was suffering from PTSD....and how it plays out again at the ending is both frightening and powerful. One of the best episodes of season six...well written and very well acted as well.
By the way, this story starred George Nader, a very talented American actor whose career was cut short in the US when stories about his homosexuality made it to the tabloids. Because of this, he went to Europe and made a string of enjoyable James Bond-type films which are well worth seeing.