My impression of Mary is that she is a complex character. She changes dramatically throughout the story. It is completely unexpected.
At the beginning of the story she is patiently and lovingly waiting for her husband to return home from work. She loved him so much that she was happy just waiting for him. 'Each minute gone by made it nearer the time he would come' shows this. She is so willing to serve her husband that she says 'I'll get it!' and 'jumps up' to get him another drink even though she herself is six months pregnant and should be resting a little more. She always addresses him lovingly as 'Darling' while he is very short with her.
In the second half of the story, she coolly and unhesitatingly kills her husband. Her behaviour here is totally out of place with that of a woman who loves her husband as much as she appeared to in the beginning. She killed him 'Without any pause' and did with great violence. 'Brought it down as hard as she could' shows this. The rage she feels for her husband is so different compared to the warm and devoted wife waiting for her husband to come home earlier on. Despite the fact that she appears to be such a meek and 'placid' woman she is able to go ahead and swiftly murder her husband.
In the third part of the story she is able to cunningly plan her way to escape the law. She is even able to rehearse it 'several times more'. There is no sign of wavering and she goes out to speak to Sam the Grocer in a cheerful manner while 'smiling at the man behind the counter'
Yet Mary is not completely heartless. Although she has rehearsed her part mentally, the sight of Patrick 'lying there on the floor' makes her break down into real tears and sorrow. She 'ran over to him, knelt down beside him and began to cry her heart out'. This shows that she did have genuine feelings feelings for her husband and felt terrible over his death.
