zxcvbob while i dont disagree with your statement about number 8 birdshot i will contest that in house most hallways and rooms are no longer than 10 yards. and at 10 yards no choke my #8 is a good enough pattern to be lethal.
It isn't the pattern that is the problem. It is the lack of penetration. In bare jello at 10 FEET (not yards), #8 shot typically will not penetrate any deeper than 5" with most of the shot stopping by 3" - and that's jello.
In a real person, with clothes, a sternum, ribs, maybe a little more meat than the typical average male (Americans on average tend to be about 1" thicker than the global average), there are a lot of scenarios where the #8 shot isn't going to penetrate deep enough to force a physiological stop. It goes back to the momentum issue that 44AMP quoted above.
For comparison, less-lethal beanbag rounds are typically just #9 shot in a cloth bag. They use such light shot because it doesn't penetrate. However, both #8 shot and less-lethal can and have killed people, so you don't want to use them unless the situation justifies deadly force - and if the situation justifies deadly force, then #8 is a little light for my liking.