Scorpius goes back not because it is right or easy, because it is neither of those things but it is something that he can actually do. People talk about Dumbledore and what he would have done and Scorpius laughs hollowly and reminds them that Dumbledore is long dead, his very bones blasted to dust by Voldemort before Scorpius was even born. He has grown up in this war and he refuses to think of it as anything else because Voldemort may have declared himself victorious but his father always kept him out of public and never let him read the paper. It is not until he reaches Hogwarts age that he realises people are still dying every day because the resistance won’t give up. He remembers classes in the Castle, cold and cruel, and always too few children in classrooms that dwarfed them because so many others should have been there with them. Scorpius does what he told when it is told to do it for so long that he almost forgets how to do anything else and then his father is killed by Voldemort himself and everything changes. Draco had only ever served Voldemort and even he is struck down; his pure blood doing nothing to save him. That is what makes Scorpius pack his bags and run until he finds them.
He doesn't expect the rebels to trust him as such but he did expect to be treated a little kinder than he is. He can imagine many that seek them out turning tail again and running if they lived the sort of luxurious lives that Scorpius did and their nerve fails them. Being squashed into a small room with four other young wizards and only really being spoken to when one of the 'Order' are pumping him for information makes the first few weeks are miserable. Yet it has been a while since they have had one defect so soon after Hogwarts and his mind is a goldmine of information on how young witches and wizards are trained to think by the school before the real world gets a hold on them. He thinks maybe they were hoping for something more from him but he doesn't know what. Hope maybe.
Hope is what he left for. Hope is what they are meant to already have and things just seem desperate. The world is gray and cold, food scarce and drink abundant for those who have the coin. Food is hard to get a hold of no matter how much gold you have in your pocket but alcohol always finds a way and it is poisoning both sides from the inside out. It had already killed his grandfather.
Drastic action seems the only way Scorpius can make any sort of impact because it is not like he can go back home now. They would kill him, and probably already think him dead. To turn up alive would be to reveal everything without saying a word. He can't stay either because within six weeks another who 'turned' has been branded a traitor and killed by the so called good guys without them having any sort of real proof. The world has gone mad and so doing something crazy doesn't seem stupid anymore. He can do something by stealing the time-turner from the family vault and going back to before this madness began. School taught him all about the dangers of messing with time but he doesn't see how he can make things much worse than they already are. He doesn't even worry about erasing his own existence because he has only ever existed he now realises, never truly lived and living is what matters. There is that and there is also the fact that while trying to change everything might seem selfless to some it is also very selfish. Scorpius knows he could create a world far worse but that won't be his problem anymore. It might be a problem for a version of himself but not for him. His misery can end if he fails and it doesn't matter. He's grown up alone, never trusting and surrounded by the fear of betrayal and so he fears nothing. Scorpius grew up being giving everything and then realised it meant when it came to important things he has nothing. Not even his own bedroom is safe. Not even his own mind. He has to do something because he doesn't want the rest of his life to be nothing. Nothing that matters. Nothing changing. Nothing worth living for.