The Dauntless faction is split in two. Those labeled as "loyal" Dauntless are allied against Jeanine. Those who are labeled as "traitor" are allied with her. Of course, we doubt the traitor Dauntless think of themselves as traitors. It all depends on which side of the line you're on.
Those on the same side of the line as Tris are Lynn, Marlene, and Uriah. Some of them have issues. Uriah is upset that his brother Zeke is a traitor. Then it turns out that Zeke isn't a traitor. Whew, that was an easy conflict to resolve. Marlene likes Uriah, and this makes Lynn jealous. Does Lynn like Uriah, too?
No, Lynn doesn't like Uriah. Lynn likes Marlene. Life kind of stinks for Lynn, who seems to be the only lesbian in the whole world—and she doesn't seem to have any decent friends to talk to about it. She's looked down on, judged by Tris for her short hair and baggy clothes, and other people think she's just angry at the world. It doesn't help that her sister Shauna is shot, her brother almost kills himself during a simulation, and Marlene dies during the same simulation.
Basically, Lynn has to deal with the most crap of anyone in the whole book. And what does she get for it? She gets to die. And she dies because her friends are stupid. Lynn is shot and can't get any medical attention because the reckless Dauntless destroyed the hospital for no reason other than the fact that they're Dauntless. So Lynn basically gets to shout out "I'm a lesbian!" with her last breath and then die.
Tris says about Dauntless, "[W]e really are the cruelest faction" (15.39). She's not talking about their nasty treatment of Lynn (she doesn't really see any problem with it, actually); she's talking about the way Eric, their leader, behaves. Eric is firmly on the "traitor" side, and because the Dauntless don't believe in any gray area, Eric shoots a child in the head, making sure we know that he is a villain.
As Paula Abdul would say, he's a cold-hearted snake.
Eric meets a pretty grim fate. Tori, Tobias, and some other nameless Dauntless make themselves new leaders and sentence Eric to execution. Talk about an unfair trial, huh? Tobias shoots Eric in the head and, well, everyone just moves on with life. There don't seem to be any consequences at all.