It turns out that while they were all playing paintball they missed some of the security cameras. That's how Jeanine was able to mind control them.
The one good thing that comes out of this is that Christina finally forgives Tris. She understands what it's like when a person is under simulation, and she understands why Tris had to shoot Will.
Tris goes to Tobias's room and lies to him, saying that she won't go turn herself in even though that's exactly what she plans to do. ("No, it's not like I have a suicidal savior complex or anything. No, I'm not going to turn myself in. That would be reckless and stupid and suicidal, hahaha, why do you ask?!")