It's possible that Rick Hughes finally got fed up with Monica, lost control and killed her. I think he could have gone to the office that night to try and make amends, but would he take a gun with him?
Whatever Monica said to Rick at the Holiday Open House really ticked him off. He slapped her right there in public. We need more information about Rick. Since he is next in line for the job, plus he's having problems with his relationship with Monica, it really makes him a top suspect, but whoever killed Monica, went there with that in mind, (carrying a gun). He is on my list, though.