Self-destruct

self-destruct 
Self destruct (Action operator) 
Causes the specified ship(s) to self destruct. 
Takes 1 or more arguments... 
All: Name of ship to self destruct. 

The self-destruct SEXP immediately sets a ships health to zero. This triggers the ship's destruct animation rather then making the ship disappear, if that is what you meant to do, there are better SEXPs.