In “The Destructors” the most likely reason the boys refer to Mr Thomas as “Old Misery” is because

Answer 1

he is seen as wealthy and privileged compared to them. The boys, who come from a lower socio-economic background, may view Mr. Thomas as someone who has everything they do not have, leading them to resent and belittle him by calling him "Old Misery." Additionally, the nickname could also stem from a perception that Mr. Thomas is grumpy or stingy, reinforcing their negative view of him.