Cannot kill - nginx: master process nginx -g daemon off

OpenResty (assumed you meant OpenResty and not OpenRusty!) is a toolset and an advanced configuration of nginx's http daemon. So it will always include a running nginx if openresty is running.

So for me your ps -aux | grep nginx results in:

...
root      8402  0.0  0.0  49984 14580 ?        SNs  Jul25   0:00 nginx: master process /usr/local/openresty/nginx/sbin/nginx -g daemon on; master_process on;
...

My config seems to include the full path, but if yours doesn't, you could try (replace 8402 with your nginx's pid):

sudo ls -l /proc/8402/exe

what, in my case, results in:

lrwxrwxrwx 1 root root 0 Sep 20 16:46 /proc/8402/exe -> /usr/local/openresty/nginx/sbin/nginx

So the running nginx binary comes from OpenResty and not the "other" nginx. You might also consider removing the other nginx to not getting confused about the correct config files...