how can they live with that
Because in some cases, they wouldn't live.
If you fire one shot at a guy who is six foot eight and three hundred pounds, that's going to do absolutely nothing. You shoot until the threat is gone. One shoot would be fine if they were aiming for the head, but they aren't. They aren't shooting to kill, they are shooting to bring an end to the threat.
This is why they shoot centre of mass (large target, lots of vital organs to bring a stop to the target), until the person goes down. the guy was still moving around on the floor, and tbf, they should have cuffed him immediately and called the paramedics.
There are some ****ing psychos out there, and one shot is almost never enough. Some people are literally like zombies and will keep going until their limbs cease functioning or they are dead.
And believe it or not, tasers require more training than a firearm, especially in the US where anybody can get a ****ing pistol.