Secular isn’t the opposite of religion either.
Nazi Germany and Communist Russia were both secular societies. This isn’t exactly a complex issue. In fact, I haven’t seen anything that you’ve posted that comes close to refuting that simple point.
And the original article pointed out quite accurately that the greatest horrors the world has ever seen stemmed from secular nations. This fact hasn’t been challenged either.
You would be better off arguing that the next great horror will likely stem from a religious nation in the form of Islamic terror, that would be a MUCH better counter argument than trying to say Nazi Germany and Communist Russia were not secular nations.
Your problem is that you have only two baskets to place things in. There is a religious basket and a secular basket. If wars aren't religious then they must be secular. My point is SO WHAT. All secular means is that it is not based on religion. It tells you nothing else.
Your argument reminds me of an old saying: "The world is divided into two kinds of people; those that divide the world into two kinds of people and those that don't".
The answer to the question I posed about basketball is simply that it doesn't matter. If the Pope plays basketball then I guess that makes it a religious matter. If the Lakers play basketball then I guess it is secular. But who cares.
Here is a quiz: Name a non-muslim country that is not secular. In particular, give me a recent historical example of a non-secular Christian nation. I think you will struggle with that.
The point is that the only wars we have had in recent history were secular. The world is basically ruled by secular nations. Should we go back to having the priests rule just so that we can have religious wars instead of secular wars? I don't think so.