As far as I'm concerned the UN kept those people there as human shields in direct violation of geneva convention rules.
The geneva convention rules clearly state that the use of human shields does not make a legitimate target immune from attack. In other words, you're allowed to kill willing or unwilling human shields in an attack on a legitimate target. The criminal liability lies with those holding unwilling human shields.
I think the UN has kept UNIFIL personnel in the line of fire so that Israel would not overrun their former bases and discover just how closely they have collaborated with Hezbollah for the past several years.
If Kofi can keep UNIFIL personnel on UNIFIL bases until there is a ceasefire, then none of us will ever see the evidence.