Mary's tears are genuine because it says,"All the old love and longing for him welled up inside her, and she ran over to him, knelt down beside him, and began to cry her heart out. It was easy. No acting was necessary. It is saying that she still had love for him and that she didn't have to fake cry because she still loved him.
Even after Mary killed her husband she still had feelings for him. Because she knelt by him crying with real tears not fake because of the love she still had for him so she didn't have to pretend when it came to this.
This story is horrific because according to the text,"“That’s a hell of a big club the guy must’ve used to hit poor Patrick,” one of them was saying. “The doc says his skull was smashed all to pieces just like from a sledgehammer.” This is horrific because the when Mary killed him she smashed all the pieces of his skull this shows an element of horror death which makes it horrific.
This shows that what Mary did to her husband was just horrific and evil. Because she took and murdered her husband the way she did.