Since Frankenstein obviously knows what his creature will look like, why does he react the way he does toward the monster? Why does he react with such horror?

True, Victor knows the monster is ugly, what he didn't count on was how much uglier it would appear once alive.

I had gazed on him while unfinished he was ugly then; but when those muscles and joints were rendered capable of motion, it became a thing such as even Dante could not have conceived.