Mary successfully manipulates the detectives by putting on such a convincing show of innocence that they hardly seem to think of questioning her role in the affair. It is because they are so unsuspecting of her that they agree to her suggestion that they eat the dinner she has prepared - the leg of lamb that she killed her husband with. In this way she gets rid of the most important piece of evidence.
Mary proves herself extremely resourceful in her bid to escape from justice for the sake of her unborn child. She carefully plans and acts out her assumed role of innocence. In fact we see her actually rehearsing the part, as she practises what she will say to the grocer when she goes out to buy vegetables, just as if it were a normal evening.
Mary, then, feigns innocence so successfully that the detectives are completely taken in. However, one part she does not have to rehearse is that of feeling grief over her husband's death. When she sees his body after returning home from the shops, she gives way to real tears.
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.
However, although she feels genuine sorrow over his death, it is not enough to make her want to turn herself in.