Why not enact the law in August, September or even June when the rioters clearly intended to create chaos on October 1 and to humiliate Beijing?

Despite the spin of the Western media, the tactics of the rioters were a failure. Little support or sympathy for the movement. What you need to keep in mind is what narrative does the HK government want to present:

1) Police was forced to respond to the violent rioters, or

2) The HK government enacted a draconian law and provokes people to rise up.

Likely I said before, if the HK government really believes it is necessary to invoke the emergency law, it should be way back in June, August or running up to October 1.  The emergency law would allow the police to detain a suspect without bail or trial for a long period. That would effectively remove large number of hardcore violent rioters. Instead, the court system allowed bail and released almost all the arrested rioters. They in turn rejoined the riots again and again. The police has handled the situation admirably and effectively. Given the difficulties they were facing, they have contained the situtaion and limited the consequences. October 1 has passed without any major incident. The HK government has managed to get over the most difficult time period. Now is the time to put the onus on the rioters and expose their violent behaviors - exactly what you have described as "terrorizing and beating up people randomly, people with Chinese ascent, people who express anti rioting opinion and so on, including woman who lost three of her teeth and old man rumored to be beaten to death"

Basically, the HK government has emerged from a very difficult period without the aid of some harsh laws and unscathed. What is the HK government going to achieve by enacting the emergency law now?