“The forest?” he repeated, and he didn’t sound quite as cool as
usual. “We can’t go in there at night –– there’s all sorts of things
in there –– werewolves, I heard.”
Neville clutched the sleeve of Harry’s robe and made a choking noise.
“That’s your problem, isn’t it?” said Filch, his voice cracking with glee. “Should’ve thought of them werewolves before you got in
trouble, shouldn’t you?”
Hagrid came striding toward them out of the dark, Fang [his dog] at his heel. He was carrying his large crossbow, and a quiver of
arrows hung over his shoulder.
“Abou’ time,” he said. “I bin waitin’ fer half an hour already. All right, Harry, Hermione?”
“I shouldn’t be too friendly to them, Hagrid,” said Filch coldly, they’re here to be punished, after all.”
“That’s why yer late, is it?” said Hagrid, frowning at Filch. “Bin lecturin’ them, eh? ‘Snot your place ter do that. Yeh’ve done yer bit,
I’ll take over from here.”
“I’ll be back at dawn,” said Filch, “for what’s left of them,” he added nastily, and he turned and started back toward the castle, his
lamp bobbing away in the darkness.
(Harry Potter and the
What does the ‘should’ mean?
It’s a slightly dated idiomatic usage with overtones of “pseudo-formality”. More naturally, Filch could say…
I wouldn’t be too friendly to them [if I were you].
And more logically/grammatically, he could say…
You shouldn’t be too friendly to them.
…but he avoids that because it might be perceived as “permemptorily rude”.
Regarding the “overtones”, I think most native speakers would be influenced by the fact that…
I should like you to leave now
…is generally considered somewhat formal/starchy/stilted/dated phrasing, compared to
I would like you to leave now
…which itself is just “polite circumlocution” for…
I want you to leave now
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