I missed this the first time I read it, but to me, it seems to pretty clearly refer to creating a dementor - Quirrell doesn't understand what it means because he doesn't know about the true patronus charm.

I think it was implied that he somehow deduced that the dementors are a physical manifestation of death, possibly even before Harry's showcase of the true Patronus spell.

"No," Professor Quirrell said, sounding rather severe. "You don't tell us why, Mr. Potter, you simply tell us that we are not to know. If you wish to devise a hint, you do so carefully, at leisure, not in the midst of conversation."

Harry nodded.

"But," said the Headmaster. "But, but what am I to tell the Ministry? You can't just lose a Dementor!"

"Tell them I ate it," said Professor Quirrell, causing Harry to choke on the soda he had unthinkingly raised to his lips. "I don't mind. Shall we head on back, Mr. Potter?"

"I ate it". Eat death. Death eater.

Quirrell can't perform the true patronus because he isn't as hopeful and positive about the nature of humanity and the vanquishing of death. As dumbledore put it, he doesn't live, but cowers of fear from death.

And then, more interestingly, in chapter 53, when giving Bellatrix the death eater password:

"Those who do not fear the darkness..." murmured Bellatrix.

The snake hissed, "Will be conssumed by it."

"Will be consumed by it," whispered the chill voice. Harry didn't particularly want to think about how Professor Quirrell had gotten that password. His brain, which thought about it anyway, suggested that it had probably involved a Death Eater, a quiet isolated place, and some lead-pipe Legilimency.

Compare it to the plan Harry's dark side came up with on Chapter 81:

Say that, to set up the if-then expectation, and wait for people to understand and laugh. Then speak the fatal truth; and when the Aurors' Patronuses winked out to prove the point, either people's anticipations of the mindless void, or Harry's threat of its destruction, would make the Dementor obey. Those who had sought to compromise with the darkness would be consumed by it.

That's way too nice of a parallelism in prose for it to be a coincidence.