Is Mary Maloney guilty? What might she be guilty of doing?

Whether or not Mary is guilty of anything depends on what you are accusing her of.  

Is she guilty of killing her husband? Yes. 

Now comes the tricky part of her guilt though.  Is Mary Maloney guilty of first or second degree murder?  No, I do not think so.  In both of those kinds of killings, there is an intent to kill, often with premeditation.  I do not think that Mary intended to kill Patrick.  I don't even think that she intended to hit him over the head with the lamb.  I think that she simply "snapped" and reacted in a physical manner in order to get Patrick to stop talking. Or at the very least cause him to feel some kind of pain.  

"I've already told you," he said. "Don't make supper for me. I'm going out."

At that point, Mary Maloney simply walked up behind him and without any pause, she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.

Is Mary Maloney guilty of manslaughter? Yes.  I think so.  

Is she guilty of involuntary manslaughter?  Probably not.  That type of killing is defined as "unintentional killing that results from recklessness or criminal negligence, or from an unlawful act that is a misdemeanor or low-level felony (such as DUI)."  Mary wasn't in the process of committing a crime, so involuntary manslaughter doesn't fit right.  

Voluntary manslaughter, as defined by FindLaw, does seem to apply to Mary though.  Voluntary manslaughter is often referred to as a "heat of passion" crime.  

"Voluntary manslaughter is commonly defined as an intentional killing in which the offender had no prior intent to kill, such as a killing that occurs in the "heat of passion." The circumstances leading to the killing must be the kind that would cause a reasonable person to become emotionally or mentally disturbed."

That last part seems to fit Mary Maloney.  She was put into a circumstance that led to the death of her husband.  She is described as more than reasonable, and Patrick definitely put her in a state of being emotionally disturbed.  If Mary is guilty of any kind of killing crime, then I believe it's voluntary manslaughter.