Disclaimer: The story of Percy's redemption is NOT in the books. It is on this site. Therefore, I don't own Harry Potter.
His first thought was Hermione Granger.
As Percy Weasley walked past the long line of Muggleborns on their way to be questioned, he could only think of Hermione Granger. He'd gotten along pretty well with her for the short time when he'd known her, but that wasn't why he was thinking of her.
He thought of her because of Ron. While most people wouldn't have thought that Percy would have paid attention to things like that, Percy had noticed how Ron always sat up a little straighter when Hermione was around and how his eyes lit up when she was mentioned. He would always blush and insist that they were only friends when anyone suggested otherwise, but Percy knew better. Eventually, his little brother and his best friend would have to face facts; they were meant for each other.
This is why Percy thought of Hermione Granger when he saw the Muggle-borns who were in for questioning. All of them had stories like Hermione. They were husbands, wives, parents, friends, children, and everything else imaginable. He knew that Ron would have been heart-broken if Hermione were in that line and so were the friends and family of the people standing there, terrified. They were all real people and what was happening to them was just sick. There was nothing else to say about it. Except that Percy couldn't do anything about it.