"...as his thin hands buried..."
Maybe, "...as he buried his thin hands..."?

"toaster oven gave a sudden DING!"
Could lose the "sudden". The timer dung when it elapsed, just when it was supposed to. It wasn't really "sudden".

I don't have a problem with the first line at all, I think it's brilliant. I felt repulsed and harassed and that's what I look for in fiction: feeling through a character.

Also, it's excellent characterization.

Now, reading the rest, I see sparkling lines and lines that I don't really need because they tell me things I already know.

>>>The nagging voice dug into Donald Morden’s ears like rusty nails, burrowing toward each other, trying to reach the center of his brain.

Don't need to tell me it's a nagging voice, you showed me perfectly in the first line. The rest of the line, I don't really need, if I compare it to the one that follows it:

>>>He gripped the lip of the kitchen counter and squeezed his eyes shut,

This, I need and like. This shows me a reaction, it doesn't tell me about it. The rest of the sentence, I don't need either because I already have the image of poor Donald gripping the counter. I do like him picturing his mother's death, just not the line "repressing the urge to walk into the next room and strangle his fat mother". I feel this line is telling, and I've read it a million times.

>>>An image of her enormously saggy arms flailing appeared in his mind, her eyes bulging as his thin hands buried between her chubby chin and plump neck, cutting off her windpipe with a slow firmness that made him grin.

I like this, very graphic. You could split it in two and make "made him grin" a new sentence. Like, "He grinned". In the long sentence his reaction to the image gets lost to me.

>>>Donald shook the forbidden picture out of his mind and thought no more of it.

Hmmm, again, I don't really needs this. Murdering your mother is sort of forbidden already, no matter how annoying she is

This are minor things:

instead of "The timer on the toaster oven gave a sudden DING!" you could just use "dinged". I'm not a native English speaker but I think that's a verb, or it could, or it should.

All the things I've said are, of course, suggestions, nothing more.

Overall, I liked the first 13. No idea about the next 13 lines, though.