Mohamed al-Sumaidaie, Iraq's ambassador to the United Nations, accused U.S. marines of murdering his cousin's 21-year-old son, Mohammed Sumaidaie, who was studying engineering at the University of Technology in Baghdad. "All indications point to a killing of an unarmed innocent civilian - a cold-blooded murder," said Sumaidaie. "The Marines were smiling at each other as they were leaving.".
The marines acted out of self defense. 
I'm not sure why the system found it difficult.  The paragraph stated all evidence leads to murder, but there is no proof presented.  The marines may or may not have acted in self defense and they may or may  not have murdered the 21 year old, therefore the comment that they acted out of self defense is not correct or incorrect in context. 
( z1 act :ARG0 ( z2 marine ) :ARG1-of ( z3 cause :ARG0 ( z4 defend :ARG0 z2 :ARG1 z2 :mod ( z5 self ) ) ) )