This could be a really straight-forward case of jealousy and murder. Unusual for a woman to do this, but not unheard of.

But my spidey-senses are tingling. The woman under suspicion is now missing. And I hope the police are at least a little suspicious about the "boyfriend".

I hope they investigate him too, but it seems like his alibi for the time of the murder checks out (on first glance). The suspect, Armstrong, is now considered a fugitive.