Yeah I hve to agree, 1 shot 1 kill applies more to snipers exclusively. As James yeager has pointed out many times in his videos, I will echo his thoughts that handguns are not great at killing people. This is a reason you see so many reports of people being injured by handgun fire, instead of killed outright with one shot. If you had a chart of the power of each round from a .22 short all the wya up through the handgun range, through the rifle range up to say, the .50BMG, you'd see that all of the handgun rounds put together only make up about 2% of the chart. Rifles are vastly more effective at killing a human being. What we gain in concealability and portability with a handgun, we give up in power and range.
If you want a philosophy for handgun shooting in defensive situations, I'd tend to suggest the SEAL approach instead. "Two to the chest one to the head, every time makes 'em dead." It's a much more effective means of neutralizing a threat, but even then being able to actually hit a moving target understress with possible bad light, worries of civilians in the line of fire, etc makes hitting the adversary at all a challenge, let along cherry picking your target spots.
If you've never been involved in an actual shooting, no offense, but don't even begin to think you understand what it's really like. Train for the absolutely worst scenario you can dream up, then imagine real life being ten times worse.