What I would have done or not done isn't your concern. What the Dutch did and didn't do makes them party to this suit and a witness to genocide.
You asked the question - shouldn't the Dutch have some responsibility? I will rephrase my question as you are unwilling to state what you would have done. Let me put it this way - what do you suggest the Dutch forces should have done to avoid the accusation of being "witness" to genocide? Define witness by the way - in my simple mind I understand the word in its basic meaning - not implying complicity or approval of the events that occurred. I await some realistic suggestions from you as to what 400 lightly armed troops could have done to oppose a heavily armed force of much greater size fighting virtually on its own ground, constrained as the Dutch were by the rigorous restrictions placed on UN forces by their rules of engagement?