Everyone has their weaknesses.
It's certain that people have weaknesses, it's just a matter of more or less, bigger or smaller.
Zhang Yong's weakness was that he valued loyalty too highly. With him, if you wanted to get something from him, you just had to play on his sense of loyalty, and you would surely succeed.
Yang Yi didn't know what Danny's personality weaknesses were, but he did know that Danny's current biggest weakness was his family. He had a family and a business, and when a person of the underworld has both, it makes them a sitting target, no matter how powerful they become or how big their business grows, it can also become their vulnerability.
And Danny's weakness had already been tested. The Destroyer had managed to force Danny to give up his resolve to fight to the death, and wasn't it because he had a family and a business, which made him eventually give up the pursuit?
