I can't stop laughing at all the people who are trying to find ways to say that you can strangle someone with a garrote in self defense, and hence by shooting the guy with the garrote you somehow shot the wrong guy.
J... I can easily come up with a number of scenarios where I am defending my self with a garrote.
First, you've been assuming all along that the initial attacker, F, had the garrote which was used against B, and then taken away and reversed with B on the top using it against F.
It doesn't have to be set up that way. F could have attacked B in any number of ways without the garrote being initially present. B could have then defended himself in any number of ways, ending with pulling out the garrote and using it to subdue F after he gained the upper hand. (It's not important as to why B had one in the first place.)
Is it possible that B needed the garrote to actually subdue F? I can see this in several ways. F may be much bigger and stronger, or a better fighter. He may be on drugs and not going down easily. It's quite possible that B needed an equalizer and the garrote was the only thing available for him to use.
I am not necessarily disagreeing with your reaction to the scenario and what you would do... I am just saying that, in the end, there may be a lot more to it than that you are taking into account. Whether that comes back and bites you in the arse is a different question.