This is a curious assumption to make, Gertie.
Do sons often murder their mothers?
Wives, sure. All the time.
But mothers? Not usually. Not unless they are real harridans.
And what possible motivation could your son have to kill you? He doesn’t stand to GAIN anything – just lose. And do you really think so little of your son that you think he would kill you?
The only explanation I can come up with is that Polonius has somehow so convinced her that he is dangerously mad and must be spoken to sternly or he’s sure to kill. And somehow that helps to diminish the terribleness of what Hamlet is actually about to do – which is murder, not his mother but, who he thinks is the king.
Which is a LITTLE bit crazy given that he just left him praying in another room.
But the heightened atmosphere of this scene is such that NO ONE is thinking clearly.
Gertrude is suddenly afraid of her own child and her child impetuously kills someone without identifying who it is.
There has to be something in the first lines of this scene that triggers BOTH of those characters’ amygdala’s hard core. When staging this, it will have to feel so taut, so like a string pulled from one end of the bow to the other, so ready for trouble that any line could be the arrow loosed.