A human truly determined to inflict harm on someone is astoundingly difficult to stop. The starkest examples I've come across are two officer-involved shootings: First was in Oregon, where a soldier going through a nasty divorce kidnapped his kids, got pulled over for speeding, got into a gunfight with the officer, injured the officer who proceeded to land a shot through his heart, then he continued shooting back, got back into the car, drove half a mile, pulled over, said goodbye to his kids, and died. The second was a man that got into a gunfight with multiple officers and didn't stop until he had about thirty bullets in him, so of course everyone assumed he was hopped up on PCP until the coroner report came back with negative results on the drug panel.
Tasers are about as good as it gets right now if you want something that can decisively stop a person, but even then they're badly hampered by any kind of heavy clothing and rain. Their direct lethality is heavily debated, but people getting head injuries when getting zapped is pretty common, not to mention deaths tied to heart conditions and being under the influence of stimulants. Their ability to stop an assailant behind any kind of barrier is completely nonexistent as well, and if you do find a technology that can decisively stop someone through a sheet of plywood it will definitely be able to kill.
Even trying to physically restain someone has a lot of room for injury and death, regardless of training and manpower, so I'm not confident that a weapon that can both compete with a gun in its ability to stop a threat and is guaranteed to not kill will happen before Tesla full self driving exits beta.