Sounds like the "True Scotsman" fallacy. "Sure these people go to church, but they are not "true" Christians. " But even if a given percentage of "Churchgoers" fall into this category, What evidence do you have that this percentage is higher in the US than in other countries?
Well, that only stands to reason because where you have a high population density, you have a higher chance of pretty much any occurrence dealing with humans. That being said, The UK has 3 times the population density of The US and still has a lower murder rate. Then let's just consider the US. Louisiana has the highest murder rate in the US per capita, yet ranks 24th in population density and ranks number 4 in terms of how religious it population is, based on the percentage that identify as Christian, say that they definitely believe in God and pray daily. Why? Your whole premise is that secularism leads to less reverence for life, why distinguish between one type of murder from another. If your argument holds, then it should hold equally for both. In fact, Says the kettle to the pot. Your whole argument is based on Cherry-picking. Above you mention high population density, but neglect to consider that it is one of the factors that increased since 1960 in the US. You bring up the 2nd amendment, but ignore that it makes guns more available and thus makes mass murder easier to perpetrate in the US than in other countries. Instead, you want to blame it on an increase of secularism; a connection you don't have evidence for.
But you did so without any real evidence that it was even a factor in increased mass murders, other than this is what you want others to believe.
Earlier in this thread you brought up the Ten Commandments. You should be more mindful of the one against bearing false witness, which is exactly what you are doing when claiming that secularists have less regard for life than "True Christians".