Suddenly I remember that the IDF had been telling EVERYONE to get out of S. Lebanon if they wanted to avoid attack. Could it be that the Israelis told the UN to get out, they said screw you, you won't shoot me, and the Israelis went ahead and reduced the target? Then Annan says well they did it on purpose, neglecting to mention that they were warned to get out. It's plausible.
It is not plausible to me that they would deliberately hit the UN people, but war is messy and you simply cannot guarantee the safety of non-combatants in a war zone.