There is a big difference, Mush. When Muslims kill “infidels” or kill their own women, they say they are doing it for reasons that are sanctioned by their religion (and culture).
When someone who might be (at least nominally or culturally) a Christian commits a horrific crime, they are not claiming that it was sanctioned by Christianity, or Christian culture. Christians do not come to their defense, because Christianity doesn’t sanction violence, period.
Further, the assumption that everyone in a Christian culture who is not a Muslim, Jew, or Buddhist must be a Christian, or that being born into a family that practices makes one a Christian…is ludicrous. I was baptized, I might even list my religion as Christian, but I am not really a practicing Christian. If I committed a crime, Christianity would have nothing to do with it.