That doesn't necessarily work... There are already strong self-defence laws in place in various US states, stand your ground, castle doctrine etc.. if you see someone legally carrying a long-barreled semi-automatic rifle then making threats to kill them if you catch them alone is a dumb idea as is actually chasing after them as they retreat, throwing stuff at them carrying on chasing them and then finally catching up with them and lunging at them...
If you're at a riot or protest and you see someone you don't like, armed with a semi-auto rifle, then it's probably not a good idea, in general, to chase after them and try to attack them, it's not that hard to do - don't go and start fights with people carrying AR15s is something most normal people wouldn't try.
These weren't normal people, every single person he shot or shot at seems to have been a criminal - whether it was someone who bummed small boys and was let out of a mental ward that morning or someone who was into kidnapping and abusing women.