Argus Filch is the Hogwarts caretaker. He absolutely hates Hogwarts students and spends as much time possible trying to punish them for dirtying the castle. When his cat (and only friend), Mrs. Norris, is found Petrified, Filch shouts, "I want to see some punishment!" (9.44). Still, Professor Dumbledore won't let him target Harry as he wants to.
The reason Filch is so bitter and twisted up inside is that he is a Squib, a non-magical person born to magical parents. Squibs are really rare. As Ron points out, "If Filch's trying to learn magic from a Kwikspell course, I reckon he must be a Squib. It would explain a lot. Like why he hates students so much […] He's bitter" (9.59). Ron is right: Filch is bitter about being a Squib. Imagine being surrounded by wizarding kids who do spells easily as you struggle to learn to hold your wand. That would drive anyone nuts; no wonder Filch is so hostile and difficult.