Fortunately, as an organization, the Weather Underground never killed anyone. However, they certainly tried.
From David Horowitz's FrontPageMag.com/DiscoverTheNetworks.org:
March 6, 1970: "three members of the Weather Underground accidentally killed themselves in a Manhattan townhouse while attempting to build a powerful bomb they had intended to plant at a social dance in Fort Dix, New Jersey -- an event that was to be attended by U.S. Army soldiers. Hundreds of lives could have been lost had the plot been successfully executed."
http://www.discoverthenetworks.org/groupProfile.asp?grpid=6808
"The bomb was intended to be planted at a non-commissioned officer's dance at Fort Dix, New Jersey.
The bomb was packed with nails to inflict maximum casualties upon detonation."
http://en.wikipedia.org/wiki/Weatherman_(organization)#Chronology_of_events
Please see the *bottom* portion of my FR Profile/Home page for some incredible details regarding the 1981 Weather Underground-Black Liberation Army-related Brinks truck robbery and triple murder mentioned in the new anti-Obama (the Weather Underground technically broke up years before this took place).
Look for the large heading:
"About the Weather Underground-related police killing mentioned in the new anti-Obama video ad" (see bottom portion of page)
http://www.freerepublic.com/~etl
And if you haven't seen the video yet, here it is:
Do you "Know Enough" about Barack Obama?
http://www.youtube.com/watch?v=m89m0pC_bpY