He didn't kill her for idiocy

From chapter 25:

The tipster had said that Bones and her young assistant were due to eat lunch in a special room at Mary's Place, a very popular room for certain purposes; a room which, she'd found, was secure against all listening devices, but not proof against a beautiful blue beetle nestled up against one wall...

"Out of my way! " Rita said, and tried to push Quirrell from her path. Quirrell's arm brushed her own, deflecting, and Rita staggered as the thrust went into the thin air.

Quirrell pulled up the sleeve of his left robe, showing his left arm. "Observe," said Quirrell, "no Dark Mark. I would like your paper to publish a retraction."

Rita let out an incredulous laugh. Of course the man wasn't a real Death Eater. The paper wouldn't have published it if he was. "Forget it, buster. Now take a hike."

Quirrell stared at her for a moment.

Then he smiled.

"Miss Skeeter," said Quirrell, "I had hoped to find some lever that would prove persuasive. Yet I find that I cannot deny myself the pleasure of simply crushing you."

"It's been tried. Now get out of my way, buster, or I'll find some Aurors and have you arrested for obstruction of journalism."

Quirrell swept her a small bow, and then walked past. "Goodbye, Rita Skeeter," said his voice from behind her.

As Rita bulled on ahead, she noted in the back of her mind that the man was whistling a tune as he walked away. Like that would scare her.

She was set up, given a chance to realize her stupidity in attacking Quirrell and make amends, failed despite Quirrell's superiority (demonstrated then and there), and he began to enjoy thinking about killing her for her idiocy.

Her disguise wasn't very good, actually. More cleverness might have saved her.