It's not a lack of compassion, I don't think, to say that Josh committed an act of evil. We have to look at the facts.
And there are no "rules" for miracles. The miracle would have been if he had decided not to kill his boys that day. But that's not what happened.
I don't really want to debate religion with you here, but as a Christian I can tell you that someone who planned out a murder suicide for his own children did not have a last-minute conversion, or he wouldn't have lit that match and blown himself up along with them. His last act on earth would be considered evil by most cultures on earth, religious or secular, since killing children in no way helps the human race.
ETA: Christians believe that evil exists and that people have to feel remorse in order to be washed clean of it. Josh has never shown any remorse about his missing wife, and his last act on earth was totally selfish and cruel, not just to his boys, but also their extended family and the social worker who watched them go up in flames. Josh made a clear choice to do something evil - that was his choice.