I don't think it's too simple. There are people who go crazy, a combination of their psyche and things happening in their lives. The investigation may shed more light on that. Oh, and tragic as this all has been I am not sorry we don't have the deathpenalty here in Holland.
This was a very hypothetical discussion, but I am suprised people assume HH Berhard jr. would have become regent. As far as I know no law states it should be the next in line - or even a royal(!!) - in case of an underage queen/king.
Is it certain a pregnant woman was killed? I have not heard that from any of the authotities; somehow I think they would have then said "A woman and her unborn child died".