From 1x08, we have the life of Douglas "The Professor," a werewolf kept in a basement for fifteen years by the Boston vampires, forced to fight other werewolves on the full moon for their entertainment. He gets his nickname from the fact that his captors allow him to study and document the bodies of those he kills, which he does in the hopes of finding a cure for himself, but he has otherwise completely given up on ever getting out, believing that he's too dangerous for the human world. He's an old man, he's lived his life out in a cage, he's routinely inspected like an animal by vampires looking to see where to place their bets, and he could well have been made to kill over a hundred people for sport, but when he first shows up he still bothers to ask Josh's name. It's a major combination of Nightmare Fuel and Tear Jerker, despite the fact that he's only a one-off character.
Aidan's staking of Rebecca. The image of her body falling to dust in Aidan's arms was beautifully heartbreaking. Whether or not you liked her, it was hard not to feel sorry for her.
I hated Rebecca with a passion. But, when she finally died, I said aloud: "That wasn't as satisfying as I'd thought it'd be."
Which is probably one of the points of the show. Violence begets violence and it's usually not as satisfying as one would think. Looking for something to fulfil that satisfaction, to feel 'better' is a fool's errand. There are some pains you just simply have to endure and survive... and forgive.
In flashback we see Aiden as a father and a husband who clearly loves his family. We also know that becoming a vampire dosn't actually change your personality. Fridge Horror kicks in when we realise that Adien probaly watched his family die (at least from a distence) and later he became one of the worst of his kind.
Nora miscarries the baby, and as she's lying on the ground, Josh in wolf-form immediately stops raging and lies down next to her, looking as concerned as a wolf-monster can. Even a wolf will protect his mate.
More so when you figure out that the likely reason the miscarriage happened is because the baby was mature enough to transform.
Sally's heartbroken reaction to finding out that Nick is dating Zoe.
And Zoe's reaction after finding out that Sally killed him.
Aidan's tearful goodbye to his roommates late in Season 2, even though it was a 10-Minute Retirement.
Suren struggling not to cry while seeing Aidan being dragged away by Mother's goons.
Everything that happens during the eclipse.
Julia: "I loved you enough, Josh. You just never believed it." She then walks through her door.
Seeing Sally's corpse for the first time.
The conversation between Aidan and Sally as she holds a stake over him in 3x04.
After Josh finally got rid of the curse, he is turned again by Liam.
Stevie's second death, compounded by Sally's fear that she's next.
Sally's tearful deathbed exchange with her friends before she dies her second death.
In 4x02, it's revealed that after becoming a vampire Susanna killed Issac, her and Aidan's son
Sally's THIRD death in the altered timeline, after she and Aidan had become a couple.
"All the years that I've been alive... these last few, they were the best."