And they were all completely justified within their own minds.
And why?
Various reasons; various, contradictory reasons.
And why else?
Humanity would have to be entirely right in their own persons or surely we would be a cess pool of insanity.
Wouldn't we?
Thus, Minerva McGonagall decided that what must be done must be done.
Following the death of Albus, she hesitated. This had been his idea; his spell; his little project that could very well turn the tides of this war that happened to look as though it wouldn't be going so well for the Light.
Further following the death of Albus, she had forgotten of this comparatively small problem. And why shouldn't she have? Her mind was occupied with other things. Fighting with emotion; battling to keep her school open.
And when things calmed down-!
It really was for the greater good. But how could she tear a mere child away from her life and everything in it? Especially only for the sake of a vain hope?
And the minister's parting words to her, spoken harshly and yet so true;
"We are all completely justified within our own minds, Minerva."
So perhaps she could not see the reasoning behind this plan... But Albus had. And if Albus, so cautious, had allowed himself the possibility, why shouldn't she believe in him?
Why not simply ask the girl?