The IDF has business to be at the border. A lone Palestinian has no business by the border because there is nothing there. He is not guarding the border, he is trying to sneak into Israel. There IDF soldiers were doing routine guarding by the border. It is a murderous act by the Lebanese army, or by some idiot who was holding a gun in the Lebanese army.
4 UN peacekeepers injured in Sinai and taken to hospital in south Israel (Haaretz)
from the article: Israeli officer killed in clash on Israel-Lebanon border