I only recently saw this case.
Did not take me long to come to the conclusion that Mary had next to no self esteem. Matthew likely was difficult to live with, perhaps a perfectionist and could be critical.
The family may also have had some financial issues. Mary was likely looking to boost her self esteem by contributing to the family finances...Matthew would be so proud and perhaps there would be a little more money around for some of the things she wanted to have. I am guessing Matthew controlled the purse strings and watched closely.
Instead she was depositing fraudulent cheques and then writing cheques on the fraudulent money. Not only would her value not be raised in Matthew's eyes...this is something she would never hear the end of... time to act on all the visions she had of her "ugly" coming out.
I initially figured she was probably not very bright...I have reconsidered that though based on the fact she was able to kill Matthew, seemingly get rid of the cheque fraud problem and also get her girls back....I think everyone underestimated Mary...something about her I guess.