Oh, Scorpius, I didn't say anything about those who aren't pureblood at all. This is what those wars have done, they've made it impossible for you to even talk about It isn't about them. It is about the fact that as purebloods we are born into wealth and influence and comfort that others aren't, and that is very pleasant for us but it also gives us a duty. To help the poor and unfortunate, to safeguard tradition, and to protect our society and all its people from harm. To ensure peace and prosperity - and that duty is far more important than any single one of us as an individual. Of course other people can choose to do this work, but for us it is a responsibility.
Take the true scenario. You return to your own time, with the knowledge that if you kill one person there, thousands of lives will be saved, and while the future will then be uncertain, at least one horrendous tragedy will have been averted. Of course it is a horrible thing to have to do, but are you telling me that you would not do it?
If there is a war here then we will have to see that the magical community is protected, and help where we can elsewhere. Nobody wants that outcome, but we will do what we must.