Gemayel's assassination is almost certain to deepen the political crisis in Lebanon, which started after the resignation of six pro-Syrian ministers, two from Hezbollah, hours after all-party round table talks collapsed.
Gemayel's assassins succeeded in assassinating him.
From the context, we know Gemayel was assassinated.  Gemayel was assassinated by someone, and someone who assassinates in an assassin. Therefore, it is definitely correct to say "Gemayel's assassins succeeded in assassinating him."
( z1 succeed :ARG0 ( z2 person :ARG0-of ( z3 assassinate :ARG1 ( z4 person :name ( z5 name :op "Gemayel" ) ) ) ) :ARG1 ( z6 assassinate :ARG0 z2 :ARG1 z4 ) )