I got sprayed full across the face courtesy of the Connecticut Air National Guard. I had to strike a bag with a practice baton for 60 seconds, then handcuff a suspect. By the end of that minute, I was almost completely blind. In real life my options would have been to run or shoot, and if I did shoot, I would be completely justified. OC spray is no joke.
The problem with lethal force here is that in my case, my ability to fight back deteriorated over the course of that minute. I stood a much better chance of achieving subject compliance during the first 15-30 seconds. After that, I would be a whole lot more inclined to shoot, but now I can barely see what I'm doing, so good luck to bystanders or my partner once I start firing!
The trick is to shoot before you can't anymore, and that's a huge gray area.