Lots of great info and advice in this thread.
Training is key to understanding when and when not to use lethal force. Police forces regularly train on this exact scenario to keep all their officers sharp and ready for what comes their way on the street.
Another option if you're having second thoughts about using lethal force is to carry a stun gun/Taser.
While the stopping power is significantly lower than a firearm, you can "shoot" someone and not have the lasting psychological effects of using lethal force.
Keep in mind that someone hopped-up on drugs probably won't be phased by the use of Taser type weapons.
Additionally, Tasers and other stun type weapons may not be legal to carry in all areas you travel so a pre check should be in order if you choose that route.
Good luck on your choice, and pending surgery, going forward...