depressed(john).
buy(john,gun1).
gun(gun1).

kill(A,B) :- hate(A,B), possess(A,C), weapon(C).
hate(W,W) :- depressed(W).
possess(U,V) :- buy(U,V).
weapon(Z) :- gun(Z).

?- ( ebg(kill(john,john),L),
     write(L), fail
   ; true ).