He was stunned.
Tom Riddle was a man who was confident and had charisma flowing from his every pore. Though he was half-blood and returned to an orphanage every summer holiday - since he had no choice on the matter - he'd been able to gather quite the friendship group.
Egotistical purebloods who came from families who were even more egotistical than their children flocked to his side; he'd made it no secret within his House that he held certain views on mudbloods and other filth like that.
The whole school loved him. Females adored him and they fought amongst themselves just to have the chance to say hello. Males, though they did hold some resentment towards him for how he managed to hold every female's affections without a thought, they did respect him for his knowledge and power. The purebloods loved him for the power he was planning on giving them all.
So yes, he was stunned when a pureblood, Henry Mcdonald, turned down the chance at helping him clean up the world. (Of course he didn't word it like that, he worded it in a more political sense) Henry Mcdonald had made a grave mistake, one for which he would pay.
In his refusal, Henry had declared him and his merry band of men to be a bunch of fools who will get their asses handed to them on a cardboard platter in Azkaban if they weren't careful and he wasn't going to partake in a suicide mission. Tom had stood by silently, holding back on the urge to curse the Ravenclaw who clearly lacked sense and judgement.
He'd get his revenge though. He'd made sure blood was spilt as punishment. Whether it be from McDonald or his whiny girlfriend, Myrtle something… He'd get his revenge and Henry McDonald would always regret the day he made an enemy of Tom Marvolo Riddle.
September Event: making an enemy
Gringotts Prompt Bank: Tom Riddle (character), Riddle Era, blood (noun), He was stunned. (first line), Henry McDonald (male oc name)