They should not have left with them 2 onboard. Abuse on ground already means a high chance of disruptive behaviour in the air. The fact that the 2 were abusive, threatening and disruptive is a major security risk. The flight deck needs to stand their ground offloading the pax.
They should've made everyone leave the cabin, that way, the 2 can be separated at the gate area. And what is it with HKG and TPE airport police? Just wave a gun around and demand them to surrender. Their behaviour was already threatening and warrants using a gun as a prop.
These 2 DM needs to be blacklisted, but with the current management, that will never happen. They sound like a pair of over entitled Hong Kong couple.