Claudius starts to get real vague here. When a person starts getting vague in Shakespeare, it’s probably a signal that a murder is getting planned. I mean – sometimes it’s obvious – but sometimes when a character wants to get another character eighty sixed, he starts to get really cagey.
“As how should it be so?” is really pretty cagey. It requires following the logic from Laertes getting to tell Hamlet something to his teeth. But it does not automatically follow that that should lead to his death. Claudius is getting vague and crossing a line but not obviously.