The precise moment Vengeance plants its insidious seed inside the chink in Ka Yeung's vulnerable mind. The Poon men must have all inherited this subversive gene.
Including his beloved wife. His reassuring words will come back to haunt him.
SS: We are husband and wife. You can share your feelings with me.
At first he dismisses BB's worry that she may be Cheung's missing daughter.
How can there be so many coincidences? Indeed! The imperceptible minutest flick of doubts in his eyes belies Ka Yeung's outward calmness towards BB's question. Concealing his turmoil he inwardly acknowledges BB's deduction - the numerous coincidences she listed can't be ignored. Yes, BB could very well be Cheung's missing daughter, thus he files away that possibility for later examination.
Ka Yeung tries to laugh off BB's agitated concern but the awkwardness in his forced smile depicts his own inner turmoil over such coincidences. At that very moment, he does not know yet what to do with this unexpected turn of events.
But when BB vows she will never acknowledge Cheung as her father, Ka Yeung knows what he is going to do with that fortuitous info. The planted seed germinates.
But he has to be doubly sure of BB's strong feeling against Cheung as a possible father so he asks the pivotal question: "What if he is indeed your father?" At hearing BB's adamant re-affirmation, Ka Yeung's doubt dissipated. He will use this bit of info to trap Cheung. His wavering eyes steadied, his face hardens, his lips pursed tightly in conviction. He will take whatever means he can to avenge his father's death even at the expense of his beloved BB to accomplish his goal.
And thus with his reassuring smile and his affectionate (and a tinge of regret for what he has to do) touch, he betrays BB's trust in him. "You're such a good person, you won't have this kind of father, won't for sure!" But then, one can argue Cheung is indeed no father to BB, her father is her adoptive father.
If his conscience is ever pricked by his deception towards BB, his timely discovery of Cheung's traitorous, nefarious dealings with the Japanese conveniently removes it. His thinking: A filial son and a staunch patriot must do what has to be done, distasteful the task may be.
From earlier episodes, Ka Yeung can be violent and a bicycle is a well-tried weapon of his, and in this scene he is not above using dubious means to a way to get result.