It's very difficult for one, over 75 years after the event, to judge actions taken in desperation under the conditions of those times. On the other hand, the suspected betrayer knew that the Franks would be sent to the camps and likely murdered. Under legal standards of today, if there were sufficient evidence to prove he was indeed the betrayer, he would likely face charges of murder, and it's an interesting question whether he could claim self-defense.
It's also interesting that Otto Frank never revealed the name that was sent to him anonymously. We can only speculate as to why.
Incidentally, as noted by the lead investigator, the accumulated evedence against the prime suspect probably doesn't meet the high standard of proof needed for a criminal conviction.
A final point: I found the 60 Minutes broadcast to be very superficial. Maybe the book is less so.
Stephen Katz