I think I am satisfied with the story now, but is there anything else I should revise before posting?
Keith Ferguson made direct eye contact with the crow that was staring at him through his office-room window. He had looked away from his laptop a moment to give his eyes a break from the spreadsheet he was working on when he first noticed it. How long it was perched on his deck staring at him, Keith couldn’t tell, but its glare unnerved him. Keith banged his hand against the window, and it flew off. He went back to work.
Keith wasn’t a model citizen. As a kid in elementary, he was the stereotypical bully who would make the weaker kids fork over their lunch money. In middle school, he loved to pick fights for the hell of it. He was almost expelled in eighth grade for sending a sixth-grader to the hospital. But by the time he was in high school, Keith developed a blood-lust. Because he didn’t want to risk life imprisonment, he kept his killings relegated to animals. He found joy in disemboweling squirrels, relished in decapitating birds, and especially loved slaughtering stray cats.
As the years went by, Keith lessened his killing of animals but didn’t stop completely. Whenever he had the chance to kill an animal, he usually took it. He had such a chance the previous day when he was taking a power drive.
He was gunning it down a straight line of a rural highway on the left lane in his Chevy pickup. Keith had barely seen the outline of a bird in the right lane when he titled the steering wheel to hit it. He didn’t even know that it was a crow he flattened.
Hours went by and Keith forgot about the crow that landed on his desk. Keith had become thirsty, so he went to his kitchen to get a glass of water. When he returned, he was perturbed to see that three crows had landed on his deck, their beady eyes piercing his very soul. Keith lowered the blinds.
Soon enough Keith decided that he had done enough work for the day and shut his laptop. He went to bed half an hour later.
When Keith resumed his work the next day, he had forgotten about the crows. The window in front of Keith’s desk displayed the view of a scenic mountain, and Keith loved to watch the sunrise above it in the morning. He pulled up the curtains to see the view. But instead of focusing on the sunrise, his attention was drawn to what must have been dozens of crows that were all staring at him. Keith gasped and went to his kitchen. He intended on using the broom that was leaning against his fridge to shoo all the gawking Corvus's away. He dashed out to the deck shaking the broom wildly at the crows. Keith hoped that he’d be able to kill at least one. They had flown off before he had the chance.
Keith was disturbed, to say the least. He had no explanation for the strange behavior of the birds and was pondering possible theories in his mind. He relaxed when his attention then shifted to the mountain. The sun was peeking just above it, and the scene was truly majestic. The sky around the peak of the mountain consisted of various hues of oranges and purples.
Keith was mesmerized until he heard a distant whooshing in the distance. He looked around for the source of the sound and saw a black outline in the distance above the mountain. Keith couldn’t identify it. The outline drew closer and the whooshing got louder. Keith made out a body and wings. It was a bird. An unnaturally huge crow, more specifically. The bird was about the size of a private jet. Its flight was uneven owing to a small tear in its left-wing. It flew close enough above him for Keith to see that it was carrying something silver in its massive talons. Clutched in them was a truck. The crow dropped it dead center above Keith. The sound of his body being crushed echoed across the mountain range.
First thing's first: you can remove the bold ' ' ' format in source tab while editing. Additionally you could stand to break the final paragraph into two or more sections so it's easier to digest. Additionally, after 'Keith was disturbed' you're missing a period.
I don't see much in terms of mechanical errors. I think the crow motif is always a nice thing to revisit in a story, particularly because the protagonist is soon to ring a death knell. However I think the transition into description of the big crow and it's subsequent boulder dash is a bit too quick, or at least not built up enough.
If I wanted to really drive home the 'oh shit' factor of the bird, I would have it be that he sees it flying over the mountaintops, and assumes its either a plane or a crow that's flying at an odd perspective. Then have it come closer, and by the time he realizes what's happening, the crow is already loosing the boulder onto him.
I'd also personally detail that it let's go of the boulder at some point to further smoothly transition into the last sentence but that is my personal stylistic gripe. The story doesn't necessarily need a "rational" explanation to make it scary, but it does need more focus on that aspect of the big bird's stature to drive home the final bloody confrontation. You can even go into short detail about how maybe it looks weirder than the average crow, give it bulging eyes or flaking talons if you wish.
And a final note: I'd maybe clarify that Keith was relaxed by the mountains specifically, rewording the two sentences so it says "He then relaxed as his attention shifted to the mountain."
Well, don't have much to add on top of what William already stated. It went on too fast and too short. There was also little reason or rhyme to the whole thing. Why this stuff happens? Why would the crows stalk the guy? to hunt it? sure but he'd have to be outdoors throughout the whole thing or the birds have to try to get in... Regular crows probably might, let alone insane mutant birds.
The whole thing can be silly too, like the guy pissed off the birds by shooing a crow or something at some point and they just decided to get revenge but it has to have something to get it going.
Also, big bird with big claws, why does it need a rock? Just stab or get the guy up into the air and dump him down - they do it with regular food.