This line has the remarkable quality of sounding like a saying, as in:
Like they always say, “None wed the second but who killed the first.” Or “A stitch in time saves nine!” And so on.
But of course it is an insane saying.
To suggest that every re-married woman is a murderer is nutsy pants. But the genius of this line is that the sound of it makes it sound like a thing. Like, that old chestnut spousal murder!
Also, it’s chock full of meaning for the wider vision of the play – Hamlet’s response to it would suggest that this might be a line he put in. But why? As far as he knows his father was murdered by his uncle. End of story. No murdering required of his mother. Or perhaps this is a little misdirection – look over here at THIS impossible proposition! But – really, we all know it’s this other thing.
Oh good old husband murdering!
Happens all the time.
Except it almost never happens.
Wife murdering, okay. That’s actually a thing. A third of all murdered women in the USA were murdered by their male partners.
And when it does happen, that is, when women murder their husbands, it is almost always in self defense after a long period of domestic abuse. None wed the 2nd but who killed the first after a long excruciating period of violence and abuse that finally came to a head and she killed him to survive.
That old chestnut.