If the story was switched and Patrick killed Mary, how do you see the story "Lamb to the Slaughter" ending differently?
The facts that would add so much gravitas to the story, had it been Patrick killing Mary, are the gender of the victim, the fact that this is a husband killing his wife, and the added fact that she is pregnant. Another issue would be that Patrick is a law enforcement officer. To go against his own line of duty is one thing. To do it by attacking his pregnant wife and ending her life, and that of his unborn child, would be an atrocity.
These facts established, let's move to what would be the consequences of such acts. If Patrick had tried to conceal his crime by feeding the leg of lamb to the investigating officers (which would still be his co-workers), he may have actually succeeded. The officers are likely to tap on their cronyism and obviate cues that may render their partner potentially guilty. However, if Patrick is not smart enough, and lets himself get caught, chances are that the gravity of his crime will earn him even the death penalty.
All one needs to do is research the news and what often happens in situations such as these. The law tends to be less lenient toward males when they are the ones who perpetrate crime against females. The idea, in some cases, is that other variables may factor in when women commit crimes, such as their states of mind, hormones, temperament, and other things that may be just cliche, but still are accepted as possibilities.
Therefore, had the story been different, Patrick must have definitely suffered the consequences of the macabre act of killing his pregnant wife, unless his friends had helped him cover the crime.