Why is he insane?

Until, now Hamlet has not killed anyone and no tragedies have occurred, but Samuel Johnson has already pointed out various, one of which is expected or at least Hamlet has planned until now and that is the death of Claudius. Johnson also notes how Hamlet has done anything in this novel but sane things.

Why is this the case?

He clearly wants attention. He clearly wants to demonstrate that Claudius killed his dad, and that things are not right. But also, his attitude towards ''seeming and not being'' is a distraction to make his real mission and strike cold blooded Claudius once and for all. How he kills him still remains a mystery.