I don't recall any deaths from roof and tunnel hacking. There are serious potential hazards. There is a case to be made for securing the roof of building 54 to deter suicides. But it appears that the much stronger security measures and much stiffer punishment for unauthorized roof and tunnel hacking since the '80s were not motivated by fatal events.

You are quite correct. Apparently I was mis-remembering. After doing some digging, I found the incident I was thinking of:

In January 2006 a freshman fell through a skylight and broke several bones: http://tech.mit.edu/V125/N65/65skylight.html. This seems to be what started changing attitudes.

Here's another one. East Campus freshmen are evidently hard to kill: http://tech.mit.edu/V119/N62/62shaft.62n.html