"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.
"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.
"We take care of my mom first—she's my number one priority. I have to make sure she's safe." (15.100)
In case you haven't caught by this point, Nick's mom is his number one priority. He risks his own safety to make sure she's safe, and it pays off.