Because Kytha was interested. Set during L attending school.
Raito's decided that the reason L gets away with being so intelligent is because he's actually retarded. That's the only explanation. With his curling toes and hunched shoulders, L has all the body language of a shaved ape, so what Raito can't understand is how on earth there aren't any loopholes yet that would let him kill this man. Chimp. Thing.
Raito remembers studying a principle of evolution, the kind that spawned bats and birds. Convergent, that's what they'd called it. Where two different species raced each other in the morphology slots, reaching the same conclusion if they were unlucky and crowding the other out if they weren't.
If evolution were on Raito's side (and it surely is by now, evidenced by the Death Note) then he should be on top already: speeding along on the wings of his genetic superiority, empowered by DNA that keeps his fingers from a magnetic attraction to his mouth. Sentience, in other words, or maybe just social awareness. Innate, blessed understanding of why it's not considered proper to touch yourself in public. That kind of thing.
Which L lacks. Raito can't stop watching the way L's fingers balloon out his cheek when L gropes for a piece of chewed parsley in the back of his throat. Knuckles wet, slimy with bovine contentment. Eyes rolled down with placid consideration of the carpet until finally Raito wants to reach out and pull L's fingers away, make L look at him instead so he can hunt for a sign of intelligence in those moist, dark-ringed eyes.
In the meantime, Raito stares, and seethes at evolution.
Primates and a certain breed of marsupial both have opposable thumbs, though they lack comparative genetics. For all Raito knows, L might secretly be an overgrown trichosurus vulpecula, employed in a masquerade of humanity. Or a koala. Soft, pliable, and--ultimately--baffled by the world.
Raito isn't sure. L is so good at what he does that Raito finds himself second-guessing everything in a whirlpool of calculations. It's difficult to plan out his next Kira-targets when so much of his life is taken up by this pasty-limbed boy, studying him relentlessly, driven for proof.
L swings from stupidity to godlike intelligence like a pendulum in Raito's brain, passing by idiot savant at the midpoint of each arc.
Fresh each morning, Raito decides that L is secretly so gifted that his behavior is all an act. The connections are there: the ability for L to deduct that a name and face were both required for the Death Note to be used, his percentage pinpoints that leave Raito as a suspect on the case. By evening, flush with a day of watching L fumble with manual dexterity and pluck at his lower lip, Raito dismisses all events as chance. L is a mere beneficiary of fortune. That's even what his name must mean--L for the English word of 'lucky.'
But if that's the case, then that's a clear sign that the detective is baiting Kira, which means that he is doing everything on purpose, and by the time midnight rolls around, Raito's frowning in his sleep.
It's crazy talk. Self-destructive. And it forces Raito to research--briefly--the correlation between heightened intelligence and paranoid schizophrenia, doubting his own considerable talents when every piece of evidence points to the chance that L might be smarter than he is. Impossible.
Afterwards, for one whole day, Raito wonders if L just wants to make him think that the detective's trying to drive him insane. If so, L's doing a stellar job. Grade A-plus.
L, Raito discovers at the vending machine one day, doesn't carry any change in his pockets except for one fat 500 yen piece at a time. "The jingle," L explains, pulling out the coin and holding it up pinched between fingerpad and thumb. "If Kira is investigating, the noise is too easy to follow. 500 yen is enough money. The rest is in bills."
L says this after Raito asks to borrow 200Y to buy a drink. He'd been hoping to palm one of the coins and fingerprint it later--L's real name must be in a crackable database somewhere if he's desperate enough--but 500Y pieces are too wide and Raito can't perform the switch while L is watching.
He selects hot coffee. L watches him drink it, and then refuses a sip.
Wherever Raito goes, L is always one step ahead. It's uncanny. Raito asks Ryuuku that if death-gods are real, maybe psychics are too--maybe L is telepathic, but not enough to actually confirm Raito's guilt. Ryuuku only grins. Not telling you that, he leers, and then wraps himself around a chair in search of an apple core to ravage.
Raito rolls onto his stomach and bites his own thumb out of frustration before he realizes what he's doing. He withdraws the digit. It glistens with primate spit. He compares the shape of it to L's--skin tone, joint--and then wipes it clean on his pillow.
Maybe, just maybe L's deficiencies are compensation for a talented brain.
Except that Raito's got everything L has. In fact, he has it better. He knows how to put his clothes on properly in the morning; he knows how to tie his shoes, textbook loops and the rabbit around the tree. He knows how to wear shoes in the first place.
And to round out his numerous qualities, Raito has girlfriends. Or at least the ability to attract female companionship; Raito likes his hobbies, girls counted among them, and when he sneaks a glance at L's desk halfway through a biology lecture, L's busy looping ugly circles on his paper instead of admiring the teacher's ass.
Raito excuses himself afterwards to study in a cafe. L follows. The wet, sucking sounds of the detective's mouth on his teaspoon are distracting. Raito keeps jerking up his eyes to L, and each time, he sees L staring fixedly back.
"Let me borrow it," Raito says eventually, breaking the silence. Then elaborates. "The spoon."
L hands over the plastic; for a split-second, Raito touches the teaspoon gingerly, not wanting to mess up any latent prints until he realizes that L's tongue has probably sopped up all the oils. Resigned, Raito plonks the spoon into his own cup and leaves it there. When L reaches for the plastic knife next, Raito deftly claims it first and uses it to slowly cut his bagel into tiny chunks, first bite-sized and then smaller.
Stripped of his tools, L resorts to sticking his fingers back in his mouth. His eyes remain plastered on Raito's face. Raito can see the detective in the peripherals of his vision, big black holes where pupil and iris merge, hungry pits that waver between cluelessness and enlightenment.
L gnaws on his knuckles. He leaves tiny divots behind when he's through, stapled imprints where his teeth pressed into his skin and gifted him with rows of dental identifiers. When Raito turns his head in dismissal from now-inedible bagel shreds, he glimpses the signature of L's genetics printed on the detective's thumb.
When Raito looks up, L breaks into the goofiest smile the world's ever seen.
Then he reaches over and steals the spoon back.