The break-in sequence in the beginning. Clyde is forced to watch his family killed while he is tied up and helpless. Darby, being a Complete Monster he is, kills them one by one in cold blood, for no reason other than enjoying it, with a single stab each (just by pure chance missing Clyde's liver and thus allowing him to survive), before taking Clyde's daughter to another room. It's not shown what he did to her, and probably for the best.
The death of Ames. What was supposed to be a relatively peaceful death by lethal injection turns into something horrifying because one of the chemicals was switched: instead of painkiller, he receives a solvent that burns his veins from the inside out until blood begins pouring from the holes in man's chest.
The death of Darby, beginning with Clyde's To the Pain speech beforehand. He then starts by using a circular saw to cut off his leg while Darby screams as much as he can. Bonus points for Clyde sending a video-recording of the procedure to Nick's house, and Nick's ten-year-old daughter sees it. Her reaction afterwards says just how bad it was.
The crime scene; blood is dripping into buckets that are almost full already. On the operating table is what is left of Darby; he's missing all four of his limbs and his head, which is sitting on a nearby table, and his tanktop is practically coated in blood. Even the two detectives almost puke at seeing this.
"Good news, Counselor: we found Darby. I gotta say though, he's looked better."
Clyde later confirms that cutting off Darby's limbs was one of the least painful things he did.
Clyde himself is this, with a healthy dose of Paranoia Fuel. Imagine someone whose job it is not just to kill people, but kill anyone. People who are almost impossible to track or get close to. People who trust very few others and are surrounded by cordons of security. Now imagine someone who has spent years, if not a decade or two, thinking "how best can I kill this person"? That's all he was trained to think about. Mission after mission, body after body, death and assassination in every form imaginable (and some rather hard to imagine) were all he concerned himself with. And now he has a reason to turn all that against the system that trained him: once he gets over the initial despair, nothing's left but Tranquil Fury.
When Nick confronts Clyde about it, and asks if all that was just for revenge, one of the few times when Clyde is visibly furious follows. He had ten years, and could cause the deaths of anyone involved at any time: if he wanted to, he'd kill them all overnight, simultaneously, efficiently and probably staging it all as accidents. Nick lives because Shelton allows him to live, out of desire to make Nick see the error of his ways; even if it takes forcing him to endure all what Clyde went through. Including being Forced to Watch helplessly as his friends die before him. And if that wouldn't be enough, it's safe to assume Clyde could make Nick relive the deaths of his family as well.
