Here's how I've interpreted it: behind the scenes, Dik Wan Tze wanted to eliminate all opposition before making his grand entrance onto the scene in Shanghai. With Fung Ging Yiu and Lip Yan Wong both dead, the new big men on the scene were Hui Mun Keung and Ding Lik, and these two were doubly dangerous because of their brotherhood/partnership. Each one would be formidable enough on his own; together, the brothers would be very difficult to unseat as Shanghai's criminal overlords.
Dik Wan Tze was instructed by the Japanese authorities to eliminate Hui Mun Keung, but for his own future in Shanghai, Dik must have understood that he needed to get rid of Ding Lik as well.
Dik Wan Tze must have ordered the French assassin to eliminate both Hui Mun Keung and Ding Lik in one hit. On the night of the hit, Hui and Ding went clubbing together. The assassin must have anticipated that the brothers would leave the nightclub together as well and thus, he could get both at the same time.
What the assassin didn't anticipate, however, was that Hui would leave the nightclub, but Ding would stay inside. So immediately after the hit, the assassin was quite pleased with himself until he saw that he had gotten only ONE of the brothers.
So basically, the French assassin only got his mission half accomplished; that's the reason for his dismayed expression when he saw that only Hui Mun Keung had gone down.
But he did that a lot in the original series too.I thought the flashback of HMK was good, although he did seem a little gay wiping his nose with that white tissue hehe.
A whole episode? Heh, that would have been THE BUND 1.5!If it was me directing (hehe), I would have made this flashback much longer, maybe an entire episode or two, explaining how Ding Lik learned the skills from HMK on one particular case that we didnt get to see in part I. Maybe it would also explain why Ding Lik was able to transform so suddenly later, as you mentioned.
I liked the flashback because it was so unexpected. With Hui Mun Keung dead, you didn't expect Chow Yun Fat to appear in the sequel outside of recycled footage from the first series, but here was five minutes of entirely new footage of Chow as Hui in the sequel! I was surprised and amazed the first time I saw it!