This guy may be a cool teacher, but he turns out to be one bad dude. Let's put it this way: No Dermott, no dead Amy. Yikes.
Jake describes Mr. Dermott as an all-around awesome guy:
Dermott was not one of those pathetic teachers who tried to be cool. He was the real thing. In his younger years he'd been part of a rock band, and he'd won a few motocross championships. He had a major set of groupies around school who signed up for boring crap like Comparative Religion just to sit in his front row. (5.56)
Okay, so he's awesome… except for the fact that he seems to hate Jake West with a passion. Mr. Dermott pushes the Jake-did-it theory about Amy's death pretty heavily. After Amy dies, he even organizes a bunch of speakers to come to school and talk about what can happen when you say mean things to other people. He also warns Jake to stay away from Dani so he doesn't ruin her life—you know, the way he ruined Amy's.
So why is Mr. Dermott laying all this guilt on Jake? At first we think it's because he really cared about Amy. He was her counselor after all, so she might have told him how much Jake hurt her feelings. But in the end, we find out that Mr. Dermott tried to shine the spotlight on Jake so no one would suspect his role in Amy's untimely demise. Needless to say, Jake is pretty ticked off when he finds out:
"You caused Amy's death?" I could hardly believe I was saying it. "After everything you did to make me feel responsible for it, you were the cause of it all along." (35.36)
Dani has a pretty positive view of Mr. Dermott right up until she finds out that he's the one to blame for Amy's death. That's probably because Amy writes such glowing things about him in her diary. And, truth be told, Mr. Dermott does seem genuinely nice and involved with his students. Especially the pretty ones:
Now, standing outside the classroom, I badly wanted to give [Amy] my assessment of Mr. Dermott, her Mr. Knightly. I would tell her she knew a romantic hero when she saw one. The guy was dreamy, smart, and confident. And the fact that he'd come to her rescue that day made him close to perfect in my mind. (12.1)
All that dreaminess is hiding a really gross interior. Mr. Dermott might be really kind to Dani and extremely interested in her, but why? Is it because he hopes that if he makes her his friend, she'll never suspect him of hurting Amy? Or is he looking to make the cute new girl his next student-girlfriend/victim? This guy is super manipulative, so we wouldn't put anything past him.
Dani discovers Mr. Dermott's true colors when she talks to him in his office about Amy. He refuses to give her any meaningful information and seems sort of surprised when Dani mentions that Amy kept a diary. Later, though, he breaks into Dani's house to steal the book and make sure there's no incriminating information about him in its pages. Seriously—this guy is super bad.
This brings us to the crux of Mr. Dermott's badness: He's clearly not above seducing his students, especially ones with negative body images and very few friends.
Think about it. Mr. Dermott is pretty much a classic predator. He builds up a relationship with Amy and makes her feel like they're friends, maybe even that he's her only friend. He pays special attention to her when she's feeling slighted by her classmates:
Mr. Dermott was my Mr. Knightly today. We had to pick partners in history. Everyone had a partner. Even my one true friend, Katy, was asked by Brian Folger to be his partner. She looked at me sort of pleadingly, and I nodded for her to go with him. I mean after all what kind of friend would I have been to say no. She really likes the guy. Everyone coupled up and yours truly was sitting alone on her massive butt with no partner. And it wasn't even kickball. I mean, I'm terrible at sports, but I'm pretty damn good at writing a history report. So Mr. Dermott walked over, pulled up a chair, sat down with that heart-stopping smile of his, and said "Well, I guess it's you and me kid." Then he leaned closer and said "Frankly, I think I won on this deal." Don't know what soap he uses but he smells heavenly. The rest of the girls looked jealous. (10.11)
This makes Amy feel important and wanted, like this super special and cool teacher wants to be with just her. Mr. Dermott also sees her for counseling sessions and uses his inside information about her to get something going with Amy. Amy's unpopular, friendless, feels unattractive and isolated, and is going through a major family crisis after losing her mother. The poor girl is desperate for some attention and affection. When it comes to Mr. Dermott, she never stood a chance.
In the end, Mr. Dermott shows his true jerk colors once again when he's confessing to being there when Amy died:
"It was all her fault. She claimed she was going to tell the school board. I went to stop her. I had no idea she would swerve off the road," Mr. Dermott blurted. Sweat was pouring off of his forehead, and his eyes nearly popped from his face. "I tried to climb down to help, but the car burst into flames. It was hopeless […] It was just a brief affair. It meant nothing." (35.33, 35)
That's pretty stone cold. Not only does he say that their tryst meant nothing to him, he blames Amy for the fact that she died. If she hadn't tried to tell on him for committing a crime, then he wouldn't have had to follow her in his motorcycle, watch her drive off the road in a panic, and then run from the scene of the crime. Clearly, Amy's the one to blame.
Mr. Dermott might be a villain, but we wonder why he decides to confess everything to Jake. Could it be that his lies just finally get to him? Or has he gone so far (breaking into a student's house) that he knows it can't be long before he's caught? When Jake spots him at the end, the guy just looks unhinged. It's a fitting look for someone with so many skeletons in his closet.