We don't know for sure if Mary does have a guilty conscience. However, if she does, then she doesn't really show it. Although her killing of Patrick is carried out in the heat of the moment, it is still a crime all the same. And Mary's subsequent behavior doesn't suggest that she feels any remorse for what she's done. She meticulously sets about covering up her crime and establishing an alibi. There's the coldly methodical way she concocts a story for the police. Then, of course, there's her novel method for disposing of the evidence. Again, one would have to say that this doesn't appear to be the behavior of someone with a guilty conscience.
Besides, if Mary really did feel any guilt or remorse for what she's done, then she has ample opportunity to confess to the police when they arrive. It's clear from all the available evidence that this is not a premeditated act; no charge of murder would've been filed against Mary as a consequence. Yet she chooses not to confess, and this, in conjunction with all the other evidence, indicates that Mary doesn't really have a guilty conscience at all.