Quote 1
I should think there never can have been a man who enjoyed his profession more than Mr. Creakle did. He had a delight in cutting at the boys, which was like the satisfaction of a craving appetite. I am confident that he couldn't resist a chubby boy, especially; that there was a fascination in such a subject, which made him restless in his mind, until he had scored and marked him for the day. I was chubby myself, and ought to know. (7.7)
Mr. Creakle is obviously a sadist. He "had a delight in cutting at the boys." He loves whipping boys so much that he feels "restless in his mind" until he finds a new victim. But it's also worth noting that David remembers Mr. Creakle so vividly and fiercely because of the suffering he brings – the reason there's so much pain and sorrow in this book is because it's these things that we remember. And David Copperfield is supposed to be a memoir.
Quote 2
Poor Traddles! In a tight sky-blue suit that made his arms and legs like German sausages, or roly-poly puddings, he was the merriest and most miserable of all the boys. He was always being caned—I think he was caned every day that half-year, except one holiday Monday when he was only ruler'd on both hands—and was always going to write to his uncle about it, and never did. After laying his head on the desk for a little while, he would cheer up, somehow, begin to laugh again, and draw skeletons all over his slate, before his eyes were dry. I used at first to wonder what comfort Traddles found in drawing skeletons; and for some time looked upon him as a sort of hermit, who reminded himself by those symbols of mortality that caning couldn't last for ever. But I believe he only did it because they were easy, and didn't want any features. (7.12)
Poor Traddles gets the worst of Mr. Creakle's brutality because he's plump, and Mr. Creakle likes beating fat boys. (So horrible!) But he also has this interesting coping mechanism of drawing skeletons all the time. David thinks, at first, that these skeletons have a huge symbolic meaning: that Traddles seeks comfort in the fact that all of our suffering will eventually end (even if it's in death).
Quote 3
It was, properly, a half-holiday; being Saturday. [...] It was the day of the week on which Mr. Sharp went out to get his wig curled; so Mr. Mell, who always did the drudgery, whatever it was, kept school by himself. [...] I recall him bending his aching head, supported on his bony hand, over the book on his desk, and wretchedly endeavouring to get on with his tiresome work, amidst an uproar that might have made the Speaker of the House of Commons giddy. Boys started in and out of their places [...] boys whirled about him, grinning, making faces, mimicking him behind his back and before his eyes; mimicking his poverty, his boots, his coat, his mother, everything belonging to him that they should have had consideration for. (7.32)
The boys are so horribly treated by Mr. Creakle that they seize every opportunity to act out against other people when they have the chance. Suffering doesn't necessarily make you more sympathetic. In fact, the brief freedom that the kids get from Mr. Creakle make them torture poor Mr. Mell, mocking his poverty, clothes, and even his mom eventually.