I don't think there's any particular evidence in the story which bears on the intelligence of unicorns, save for the fact that some non humanoid magical creatures such as acromantulas are much smarter than their mundane kin. This alone should be sufficient to raise it to the point of being worthy of consideration.

Here's another possibility which Harry failed to consider; the side effects of drinking unicorn blood may in fact be worse than death, not for the individual, but for society, if it does something like permanently compromising the recipient's morality. Quirrell is already amoral enough not to care, but if Hermione had been saved with unicorn blood, she might have come out like Demented Harry.

It might not be the sort of thing which is obviously likely enough to be worthy of consideration in his position, but the way Dumbledore described in in the original canon, I think suggests it as a distinct possibility.