Nick felt the color drain from his face. This couldn't be happening. They were mugging two tourists? (1.256)
Nick has violent tendencies, but he rationalizes these tendencies by only being violent toward people he thinks deserves it. These two tourists are innocent, so Nick is aghast that his friends attack them—and so he defends the tourists.
Before he could think better of it, Nick swiped him upside the head with his backpack as hard as he could. (1.78)
When Nick is stressed out or feels threatened, his first reaction is to turn violent. This will become a problem for him later on. He might have an easier time resisting the dark side if he were more levelheaded.
One day…one day he was going to get out of this hellhole. Even if he had to kill someone to do it. (1.194)
Nick is pretty desperate to change his lifestyle. Early on, it feels like this could be foreshadowing—like, Nick might have to kill someone to escape. But it turns out that he doesn't, so this line is simply characterizing Nick as a character who has no problem with violence if it acts as a means to an end.
Alan laughed as he squeezed the trigger. (2.1)
Alan is a bad guy because he takes joy in the violence he commits. When Nick was fighting at school, he didn't seem to enjoy it. That's one thing that makes Nick different from the zombies and the bullies.
On the one hand he was horrified the man had been eaten. On the other…he was strangely happy. The sanctimonious pig kind of deserved it. (4.215)
Nick always wanted his school principal gone, so he's glad when the dude gets eaten. That's a violent end for anyone, even someone you don't like, so it's a little shocking that it makes Nick happy. But violence isn't a big deal for him—and that's even before he finds himself needing to fight zombies.
Rosa grabbed the cleaver in her right hand before he could. His jaw went slack as she took the carving knife into her left and held the two of them like a pro as she faced their intruders. (7.2)
Is everyone in this book a trained assassin? Nick is shocked to see Rosa, the housekeeper, wield a pair of sharp knives like a pro. She's handy for food prep and for slicing and dicing bad guys.
Nicholas "Nick" Ambrosius Gautier
"That's exactly what I'm talking about. That's the evil that's seducing you. […] You have to let that anger go before it's too late. Vengeance always turns inward and it will consume you until nothing's left by an empty hole that nothing can fill." (16.40)
A key conflict in the book is one that Nick isn't entirely aware of. It seems that if he lets his anger and vengeance control him, then he will become a Malachai, which is this book's equivalent of going to the Dark Side with Darth Vader. Ambrose, who is Nick from the future, is trying to guide Nick away from this violent path.
Nicholas "Nick" Ambrosius Gautier
"Now take your vengeance on the ones who've mocked you. Kill them and eat their brains." (17.159)
Here we see the bad guys baiting Nick to come to the dark side. They want him to wreak vengeance against one of the bullies who's got it out for him. Nick's future pivots on this one violent act, and luckily, he doesn't commit it.
The ancient warrior and predator inside him wanted to hunt them down and kill them for what they'd done. (2.213)
The only reason Kyrian doesn't pursue Nick's attackers is that he must rush Nick to the hospital. Does this mean Kyrian is evil? The book casts violent revenge as a Very Bad Thing, and that's exactly what Kyrian is considering here. Or is this all okay because of the fact that Kyrian is avenging someone else?
They'd barely moved before Madaug fired at the zombies outside. The rocket exploded in the door, sending glass and zombie parts everywhere. Green and red ooze streaked through the night. (14.53)
Every action story needs a gratuitous rocket launcher blast. This one is overkill, and it ends up putting the team at a disadvantage. If you're barricading yourself inside a building, it helps if the building has a front to it.