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.
The fact that Yuzuru's human life sucked justifies this on his part, and from what we (don't) see of Nanami's home life she had more than a bit of the Parental Abandonment thing going as well.
Seiji Hama as well. When Mina comments that the lifestyle of a captive suits him, he mentions offhandedly that it's better than some other places he's been held that regularly subjected him to torture.
Crazy Awesome: The Earth Clan Elders. They're badass, and if their outrageous stories are to be believed (and it's implied by a few characters that they are), they wouldn't have been able to pull any of their stunts if they weren't batshit insane. For instance, only these guys would actually consider killing a shark that ate them by eating it from the inside out.
Ear Worm: The Opening Theme, Aiko Nakano's "Friends". And the Ending Theme as well, Hibiku's "Tsumeato" — for some even more than the Opening.
Moral Event Horizon: Mina before it's discovered that she can undo her infection of the five-year-old. It's still one of the major reasons behind Mina and Akira's big brawl in the middle of the next volume.
The other three pure-born vampires' plans for Mina. As the only pure-blood female and the Progenitor, only she can give birth to another pure-blood, and can only give birth once. Their plan is to find a way to artificially age her — or just wait until she grows up — and rape her until she gives birth. Assuming it's a male heir, he'll repopulate the species. Assuming she gives birth to a daughter, they'll just artificially age her and continue on until they get the Y chromosome they want. And that's before they find out about her "true form".
Of course, this brings up an important question — if they just need a male... why doesn't one of those 3, er, do it themselves?
The anime adds to this There is another Progenitor female, a "sister" to Mina, carefully kept hidden from EVERYONE. Problem is, she's also the leader of Telomere. which explains why the vampires in Telomere aren't under Mina's control. Ain't politics grand?
The "chastity test". High Octane? More like out-of-the-ground-right-into-the-frikken-tank Nightmare Fuel.
"It's a farce, and nobody is laughing".
The vampire attack in the church. Especially when the nun is literally hurled into the pack of vampires to be sucked dry. Even worse? She wasn't just being eaten.
Chapter 41 of the manga. Just... Chapter 41. Poor Angie.
Squick: Mina looks to be approximately 12-13 years old. She's much older, however.
Add to that how some werewolves are born. Only male children can be werewolves but even then there's only a thirty percent of the child being a werewolf. The only way to be absolutely certain of getting a werewolf is to have sex... with the man in werewolf form.
As of the final chapters of the manga, there may be an exception to the rule as the female werewolves are making a comeback in the form of Sanin's older sister Tatiana.
Sanin. Oh dear God, Sanin. First he was betrayed, brain-damaged, and nearly killed by the werewolf who was supposed to be his partner for life during their trial in Siberia with Akira and Angie. Then his half-dead body was found by Telomere, who forcibly injected him with chemicals designed to make him a psychotic killer. Then, Angie manages to get through to him and restore what little humanity he has left after, you know, the brain damage and chemicals... and a Manchurian Agent injects him with berserker serum again, forcing a traumatized Akira to put him down in self-defense. And of course, he's shown in flashbacks to generally be the nicest guy you could ever hope to meet. And to just twist the knife deeper, in a last remaining shreds of his sanity, puts Akira's gun under his own chin for him to do it. His final tear-filled words before Akira pulls the trigger? "I... I'm sorry... A-Akira...".
Akira himself. How about having to kill your friend when you were just a kid, then have to live with the feeling you're an animal, just to find out out that another friend who was presumed dead, shows up. And then you have to kill him.
What an Idiot: That shapeshifting weapon had Akira on the ropes, running him ragged and keeping him guessing on whose friend or foe. Then managing to mortally wound him when it cornered him. All seems lost, right? Fortunately for Akira, the weapon tries to be sadistic and transforms into Mina to add insult to injury. This however has the effect of making Akira very angry that it would mock his love for her. He gains the strength to go wolf and bite the thing's head clean off.
And then there are the people in an earlier arc who barricade themselves in a chapel only to open the door when someone they are largely convinced had been kidnapped several days ago by the vampires they were hiding fromasks to be let in.