Disclaimer: My first name isn't Joanne. Ergo, it is impossible that I am the owner of Harry Potter.
Quirinus Quirrell had always had a stutter as a child.
He had absolutely hated it. Every time he opened his mouth, people laughed. He didn't like people too much, but he had thought he would enjoy teaching.
At least, his pupils wouldn't laugh at him. He hated that sound, people laughing at him. He really, really hated it.
That was the reason he'd originally started researching the Dark Arts. He wanted to make sure no-one would ever laugh at him again.
They still did. His collegues snickered behind his back, and his students laughed about his subject. Muggle Studies, who could ever take that seriously?
Well, it wasn't Quirinus fault that Muggles weren't that intelligent and that things like electicity and fellytones were stupid. Just weak imitations of magic.
Of course, Professor Am-Perfect-White-Beard Dumbledore insisted somebody teach that subject.
Well, he'd show him. He'd show them all. They'd found a replacement - Charity Burbage, ridiculous name, and now he had one year off.
One year to find the Dark Lord. He'd learn from him, and then, nobody would ever laugh at him ever again.