ref : The Wailing Explained
He and his men throw the Stranger's body over the cliff and kill him.
He commits a sin and Moo-myung, who watches on, loses the ability to
protect his daughter.
She also mentions about her trap and that it would work only if
Jong-goo waits till three rooster crows before going home. She tells
him that he has sinned by plotting to kill and killing and hence he
must go through this test of faith.
Jong-goo willfully throws the stranger over the cliff intending to kill him. The Japanese stranger is not possessed at this moment in time... he's shown to be confused and running for his life. Jong-goo's sin is murder.. or the attempt to.
Jong-goo tries to justify that his daughter got really sick and hence he tried to kill the Japanese stranger to protect her. But regardless of the reason, the act still remains a sin.