Harry is confused. He wonders why this old parchment is telling the words that doesn't make sense to him. He's in the Hogwarts library, alone, with a piece of old parchment reveals the truth. That Snape is his father. What kind of craziness is this?
No way is Snape his father!
Sure, he knows that the words written are blurry and he thinks he sees something else. A note from his mother. Maybe, this will reveal something else about this truth.
Still, he can't believe that Severus Snape is his father? It's like finding out the bad guy is related to him. Since when?
He wishes he can erase this reveal.
After looking at the piece of paper, he see's Snape's cold eyes looking at him when he enters the room. Harry looks away because it still can't be true.