Yes, I am aware of that, rather stilted, study. What does it mean to be “compassionate” and what actions or values were measured? Anecdotally, I see no real difference between nominal Christians and agnostics, atheists and the dechurched. They all maintain their brand of compassion. And there is no shortage of assholes among them all.
But your point, that there are 250 million people who identify as Christians is very much the issue. Those of us who work in the church (I am an Anglican Priest) frequently decry the abhorrent level of religious devotion and seriousness among church goers. The reality is that many of them are not Christians, but merely retain a veneer of Christianity. Most people are nominally Christian. That is neither surprising nor unexpected. The genuine article is, in my opinion, lost to most of us. Most people are never really exposed to the Christian Faith “in spirit and truth”, but rather an amalgam of popularism, christianity and eastern influences. The term coined not too long ago to describe what most Churches practice is really “moralistic therapeutic deism” and as such bears little real resemblance to the Christian faith that won the ancient world.