He was jealous that he had no magical abilities, so he hated the students for it. He was a squib, the opposite of muggleborn, born into a magical family with no abilities.
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1 up, 7y
I know. He still hated a little too much don’t you think? But he did think harry was spreading the word that he was a squib, which he wasn’t because he didn’t really understand what a squib was. So filch hates Harry and other students even more. If dumbledore wasn’t watching and if harry wasn’t so important filc( would he tried to kill harry.