The main reason for your confusion has been missed by the two previous answers.
From being one of the most popular and admired people at the school, Harry was suddenly the most hated.
You are thinking of this as "the + adjective" when in actual fact it is "the + superlative adjective". It means Harry is the single person who is more hated than anyone else.
You are correct that "the + adjective" refers to a group of people. That's because in that case ("the poor", "the jealous", etc.) the 'adjective' is functioning as a noun.
However, when you use "the" with a superlative, it is functioning as an adjective and the noun is implied.
- the weirdest (person)
- the easiest (task)
- the most evil (villains)
The implied noun will come from the rest of the sentence. In your case, as @snailplane pointed out, it's person at the school.
One more example to contrast the two forms.
The bullied often go on to become bullies themselves.
Although many people were disliked at Rochdale Comprehensive, Bob was undoubtedly the most bullied.