Here's a little speculation.. and yes, it's totally just speculation...
as I've pointed out before I think the first 4 seals precede any 70th week.
they are a setup, of man caused disasters on the world, that I believe are designed to bring down the current world powers, opening a power vacuum
If they were to be fulfilled ultimately in a nuclear exchange between NATO and Russia, but limited to just NATO and Russia, IE the rest of the world does not get involved, so you have about 2 billion people dying around the world mostly concentrated in North America, Europe, and Russia, and many predominantly Christian areas of Africa and South America suffer from famine, a lot of the world's "Christian Nations" as the world considers them, will be largely taken out of power, and killed a large number of people as a result of their war.
The people left on Earth, in East Asia, in the Middle East and India, will be overwhelmingly non Christian, largely Islamic, Buddhist, and Hindu.
In their eyes, it will be "Christians" in Russia, and "Christians" in the US and Europe that had the nuke fight that killed so many and put the world in such a disastrous situation that they have to rebuild from.
It becomes plausible then that Christians are then targeted for the 5th seal.
I think along this lines because while it's part of popular in hollywood and Christian movies to depict a future of a secular European Antichrist rounding up Christians to execute, I don't find it very plausible for them to have a REASON where Christians are targeted.
But to the rest of the world, the countries in Europe, the US and Canada, and Russia are considered "Christian" Nations. If they go to a war and it results in a nuclear holocaust felt by the entire world? That'd make "Christians" demonized in most the world's eyes.