These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Alex Rider
Complete Monster: All of the baddies are pretty nasty in their own right, given that they have no problem trying to kill Alex despite him being a child. Two particular examples stand out from the rest though:
Julia Rothman, from the fifth book Scorpia, plans to make outrageous demands to Britain that will never be met, kill a high school of children when they are not met, and make the same demand to America with the same consequence knowing that they will meet them, and ultimately cause a war to erupt between the two countries. To demonstrate the weapon and show that she is not bluffing; she kills a completely separate group of high school football players without giving them a chance to be saved. She also manipulates Alex into joining Scorpia by getting him to think his father worked for them and was killed by MI6 when it was the other way around. She was planning to betray and kill Alex in the end.
Major Winston Yu, from the seventh book Snakehead, takes part in SCORPIA's plans to kill the members of a conference against poverty. His plan involves causing a tsunami and kill millions of people to make the deaths look like an accident, overshadow the deaths of the conference members with the deaths of millions, and to drum up sales for his Snakehead gang. When Yu captures Alex, he decides to have him slowly killed on a hospital bed as he removes his organs one by one one organ a day until all that is left of him is a husk. This seems to be his preferred method of execution, since he has people killed in the same hospital, in the same way on a regular basis. Yu is so bad that his henchmen commit suicide before he can punish them for failure.
Julius Grief, his looks identical to Alex's looks in every way, appeared first in Point Blanc, but this trope applied better in Scorpia Rising as his insane and murderous personality was shown more. Later, when Alex killed Julius, psychologists described it as Alex killing part of himself. As Mrs. Jones and Blunt put it, the part that they created and never should have been born.
Ho Yay: It seems that the author enjoys going into detail about the male characters' musculature.
Especially prevalent in Ark Angel, when the soccer player is examining himself... and spends a good deal of time complimenting his body.
Iron Woobie / Stoic Woobie: Alex has had a horrible life, but he hardly ever complains. He's remarkably composed even after Jack is killed in Scorpia Rising, save for his brief Heroic BSOD, though after that he is said to be much more cold and isolated, as you would expect.
Yassen, poor Yassen, his real name was Yasha. First his family is killed and village of Estrov is destroyed. Then he is introduced to the criminal underground of Moscow, after that he sent to work as a slave for four hellish years. He then becomes the cold assassin he was in Stormbreaker.
Jerkass Woobie: General Sarov. The man is messed up in the head, but he's lost his son and watched his country go to ruin, at least from his point of view- the everybody eating at McDonald's and wearing Levi jeans thing might not be so bad per say, but he points out millions of people have AIDS, nothing to eat and live in perpetual poverty.
Alan Blunt. Some of his actions are questionable to begin with, but in the ninth book, Scorpia Rising, he springs head-first over the line by arranging a school shooting in order to coerce Alex into taking his next mission. Said shooting hospitalizes Alex's best friend and Secret Keeper Tom. It doesn't help that what he does leads Alex into a trap set by Zeljan Kurst.
Yu planning to have Alex painfully tortured by forcing him to donate his organs to black market clients.
Razim crosses the line when he kills Jack Starbright and forces Alex to watch. This might perhaps be the moment where Julius crosses the line too, as he's the one pulling the trigger on Razim's orders and gleefully rubbing it in.
Nightmare Fuel: For what was supposedly a kid's book series, it packed a lot of this.
Point Blanc: Grief's plans for Alex are to dissect him. Alive.
Skeleton Key: Alex running into a great white shark and being almost completely helpless to escape it, constantly thinking Oh Crap and with nothing to defend himself with apart from a knife., followed by the shark being caught in Sarov's Death Trap. Then after that, we see Conrad trying to torture Alex with the mill's grinder.
Scorpia: The English reserve team literally dropping dead, and up until later on in the book, no one even knows how.
Ark Angel: A soccer player takes a shower while wearing a caesium medallion covered in wax. Due to the material it's made out of, the hot water ends up melting the wax, and then the medallion explodes and kills him.
Crocodile Tears: Desmond McCain's form of questioning Alex? Forcing him to hang onto a ladder with his bare hands as he's dangling over a pit full of crocodiles. And then he leaves him there to die. He doesn't, obviously, but the whole chapter involving this scene is crude.
From the same book, there's Gunter's death. After Alex asks for a cigarette, he grabs the pack and casually opens it. And then discovers that Alex hid a scorpion inside of it...
Paranoia Fuel: The ease with which MI6... deal with Bulman halfway through Crocodile Tears. They erased all of his financial and personal records, then made new records appear as though he was an escaped Broadmoor inmate named Jeremy Harwood who had killed Bulman. They did all that in a matter of hours. They executed his fate overnight as he slept.
Technology Marches On: Combined with Comic Book Time, with the world incorporating new technology and referencing events that occurred throughout real history.....all without the author bothering to advance the story universe more than a year. The eponymous Stormbreaker computer isn't that impressive by modern standards and is rather quickly eclipsed by the iPhones that somehow appear just a year later.
Unfortunate Implications: Herod Sayle is a Lebanese (Egyptian in the US version) villain, Alex is a white British protagonist. Could be subverted in that Herod was British, just not native.
The Untwist: In Ark Angel, the fact that Nikolei Drevin is the real Big Bad and not Force Three is made incredibly obviousnote about a page is dedicated to the revelation that Force Three kept Alex in a block of flats owned by Drevin, itís pointed out directly that no guards were watching Paul when he was a target (granted the story is from Alexís POV so itís reasonable he would notice this), and itís essentially said outright that he ordered the murder of Adam Wright. Alex even lampshades it; when Drevin is about to "reveal" this fact, Alex tells him to not bother.