riight...well, this is actually frightfully simple. Conditions in waypoints...basically, what you write in the condition field must happen before what is written in the on Activation field does. In a waypoint, not only do these two things happen but the unit of the wp will go on his way too.
Ahem, that explained, you will need to write thusly in the condition of the wp (make it a normal MOVE one, a hold one would require a synchronized switch trigger):
!alive SAM
SAM is the name of the object to be destroyed. the ! indicates a "Not", you could also write: not(alive SAM). I.e, once the SAM is not alive (i.e dead), the wp will be made true and you can go on
There you go, any more problems, ask away.
Wolfrug out.