
Mary-Kate Olsen had nothing to do with the death, as far as we know.
Here's a picture of Rihanna reacting to the news as she walked out of a performance of The Lion King.

This is the most upset Rihanna has been since Chris Brown beat the piss out of her.
According to people who may or may not know, DJ AM was distraught over a recent break-up with his girlfriend. It's also possible he was still suffering the psychological after-effects of that plane crash that nearly killed him and that dickhead Travis Barker.
Speaking of Barker: according to TMZ, DJ AM's death has caused the drummer and his on-again/off-again Shanna Moakler to stop sniping at each other on Twitter. So he did not die in vain.