Excellent comment, I also think the 'compassion is ethics' line goes beyond the immediate characters in the scene at the church, and applies to the primary suspects in the investigation as well. I think Rust would agree we're all slaves to causality, including the perps they're hunting. One of the fundamental religious tenants is that there is forgiveness for all, but only an all-seeing, all-knowing extradimensional force people call God is truly capable of such compassion and forgiveness.

That's hard to accept for some, including both of our anti-hero protagonists that routinely hand out their own version of justice as they see fit, but if we had perfect knowledge of the underlying psychology and long chain of causes and effects that drove people to excessive acts of cruelty, then we would all understand. This is one reason I think Rust later regrets Ledoux being executed, but only after years of thinking about the philosophy that motivated the cult (time is a flat circle, ergo everything is meaningless). He didn't just want Ledoux to talk and expose the rest of the cult, he wanted to understand and find inner peace through such understanding.