How did he know when all her psychiatrists (trained for years in the field) did not know? He was an engineer, he wasn't a doctor. They diagnosed baby blues. Would you blame a man who took his wife in for chest pains that the doctor said was from stress for the heart attack she later got?
You judge her by her failures. I judge her by her successes.
As for Russell Yates--who is only a series of electrons and audio impulses--we know nothing of what is going through his mind. We know nothing of his REAL relationship with his wife as opposed to the theatrical version which is produced in the midst of all domestic disasters.
We know nothing of the advice he is receiving from "well-wishers"; nothing of the "expert" opinions he is accepting as to the best way to show loyalty to his monstrous wife; nothing of the impotent guilt that is driving him to horrifying verbal diahrrea in front of the omnipresent video camera. We know nothing of what makes him seem so hollow and lacking in dignity. We know nothing of his upbringing. We don't know anything about anybody in this case, except.....
Five children were murdered, by hand, slowly, by Andrea Yates.