Reading all the reports from that book by Strassman (DMT the spirit molecule) has definetely increased my pre-trip anxiety...

But anyway, if I remember correctly, in some cases there are reasons why some people have these sort dark experiences (as described in Strassman book):

1- Some of them have been sexually assaulted during childhood. DMT can make you re-live that experience (in some cases is not necessarely bad, it can be therapeutic, allowing you to move on with your life).

2- That guy in particular from the book had low tolerance to other drugs such as LSD, and preffered mild ones like MDMA. He also had some personality issues going on.

To conclude, I´m not saying everyone is 100% free of not going to be raped by crocodiles... but it seems some people are more prone to other to experience that (esquizofrenia, personality disorders, etc).