Why do you think Ash agrees to one-on-one fight when Sing proposes to it? I always thought about it but couldn't find a good reason. What would happen if he denied to fight? I know it's won't make much difference but I can't help but think that if only Lao knew that there is no fight between them, it would have stopped him killing Ash, though it won't change everything.
I think because Ash knew, if he turned Sing down in front of his gang, it would cost him his position as his crew's boss. Sing's gang would have seen Ash's refusal as a lack of respect for Sing, as if he weren't enough of a threat to warrant the challenge, and if you combine that with the tension and pressure already plaguing Sing from his inability to explain why he hadn't yet challenged Ash over Shorter's death, it was possible one of Sing's group might have challenged him themselves for the top spot, or they simply would have thrown Sing out of the group altogether. That alone could have put Sing's life at risk. I also think Ash just had too much respect for Sing to embarrass him in front of his crew like that. It was largely about maintaining Sing's reputation and standing as a boss. Regarding him with the same respect he would have shown any other boss, by treating him as a legitimate opponent. Also, since Lao had already left before Ash accepted, I don't think him turning Sing down would have made any real difference, sadly, in whether or not Lao later attacked him. All Lao knew was that Sing requested a one-on-one battle, and therefore, put himself up for death.
But I don't think Ash ever had any real intention of going through with Sing's request, either. There's certainly no way he ever would have killed Sing. If they did fight, he would have spared him, the same way he did Arthur initially. As it was, he simply told Sing to forget it.
Anyway, hopefully that helped and thank you for the ask!
