This doesn't matter, though. Lmfao. It's not that Suu wants to kill Hwaryun just because. Suu has his own circumstances because he was working with the noble consort. She told Suu to give Hwaryun the poison cup because she has the upper hand. Suu isn't a dumbass who would outright refuse the consort because she will just kill him; thus, Suu would absolutely give Hwaryun the poison cup again EVEN IF Hwaryun played nice-nice with him. To Suu, his survival is the most important. If you were Suu, why would you do anything to jeopardize your best chance of survival? For similar reasons, Hwaryun should not have played nice-nice with Suu. If he did anything like that, then we would've never gotten this far into the story. Revealing all your plans to a complete fucking stranger is the dumbest thing you could possibly do, especially when you're a crown prince and everyone is out for your blood. I get that you're taking about the possibility of them going back in time, but I just wanted to reaffirm that the past would not have changed because the decisions made in the past were crucial to navigating through the present at that time.
To put it bluntly, I have to disagree with your claim because the notion of Hwaryun and Suu behaving differently in the past is only a null proposal due to limitations and circumstances that their surroundings impose on them, which remain constant. If I sound like an asshole, it's probably because I am one. I apologize in advance if my words offend you.
uuuumm pretty sure if they went back in time and the crown prince treated suu well and didn't keep anything from him then suu would not have handed him the poison cup