Desmond Tiny is a sadistic time traveler (and Darren's father) who is revealed to have manipulated almost all the terrible events in history just for his own personal pleasure. He brags about the way he likes to drink the blood of children, claiming "it's so sweet", and on another occasion claims he is excited to witness a volcano that will kill thousands. He pitted Darren and Steve against one another, insisting that the vampires only have three chances to kill Steve before he overthrows the Vampires. Whichever boy won-Darren or Steve-would become the Lord of Shadows, kill all of their friends, and became the ruler of the world. When Darren decides to Screw Destiny and let both himself and Steve die, Mr. Tiny instead creates dragons which results in the Post-Apocalyptic world as seen in The Lake of Souls. His motivation is he looked into the future and saw that things were going to be too peaceful for his liking, so he set the stage for a lot of chaos to amuse him.
The Vampenze Lord, aka Steve "Leopard" Leonard, is an unstable tyrant who starts up a war with the vampires to get back at Mr. Crepsley for turning Darren into a vampire and not him. Among his actions are seducing the hero's sister in order to get her pregnant; using his own son as a bargaining chip against Darren; and snapping the neck of Evra's eight-year-old son just for his own sick joy. Even the other Vampenze dislike him and only serve him because they have to.
Yellow pants with pink elephants. "Bad choice in boyfriend", indeed.
There's also the description of Vancha stepping aside to reveal "a bound, gagged, enraged Chief Inspector Alice Burgess trussed up on the floor". Darren also remarks that she's trying to speak through the gag and guesses that "she weren't saying hello or wishing us well."
Darren reuniting with his ex-girlfriend Debbie Hemlock - when she's his teacher in English class. Look at it from the perspective of her students: the new guy hugs the teacher and starts chatting to her like they're old friends.
Fandom Berserk Button: Calling it a 'British' series is bound to set off a few fans. Author Darren Shan is Anglo-Irish and uses some English slang terms in his writing (for example saying 'trainers' instead of the Irish 'runners'), so there is some merit to this. But the books were written and published in Ireland.
That said, the scene from The Vampire's Assistant where Reggie Veggie has just had his hands ripped off by a werewolf he'd been trying to "liberate" and proceeds to run around the forest screaming about his "hands, my haaaaands man, my haaaands" is right up there.
The main character just barely stops himself from attacking his beloved younger sister. That's bad enough, but he realizes that the only thing he can do to keep his family safe is to fake his own death and leave them and all his friends, forever. He's about 12 years old. It's made worse by the fact that we have to read the reactions of the distraught family.
Rescued from the Scrappy Heap: Alice Burgess, a police chief, makes her first appearance campaigning against the vampires. By the end of the series, she's one of Darren's closest comrades and has been an active fighter on the vampires' side.
After Mr.Crepsley dies, the way Darren describes him is heartbreaking.
Sam's death, especially how Darren acts afterwards.
Darren's description of his abandoning his friends and family in the first book is crushingly depressing.
When Darren said good bye to Mr. Crepsley before entering the Hall of Flames, and asked him to take his body to his hometown and burry him in his grave, in case he didn't make it, so he could be close to his family. Even Larten dropped a Single Tear.
Darren seeing Gavner murdered by their friend. Darren returning to Vampire Mountain to accuse his friend of treason. Kurda looking at Darren with a sad smile while he's being interrogated.
Book two mentions marijuana (or some other herbal drug; it's not mentioned by name) and 'shrooms.
Book eight has a character who, though appearing to be fifteen (and probably around the same stage of puberty as any real teen), is really about 27, first outright saying that he wants to date said real teens ("That's the thing I love about high school girls: I keep getting older, they stay the same age") and then trying to do... something to his much-older teacher (who is very likely younger than him.)
Plus, the books are so violent and gory, it's not even funny.