Recently I just read chapter 32-34 of the book Divergent. All the Dauntless had been injected with a special serum. Eric had said the serum could locate you. Eric had lied though, the serum took control of the Dauntless' minds. The next morning all of the Dauntless were robotic human zombie soldiers working for the Erudite to kill all of the Abnegation, everyone accept Tris and Tobias. Eric was jealous of Tobias so he wanted to make a joke of killing Tobias. Tris couldn't let him do that so she shot Eric's foot. Tobias and Tris had to run away because they were found out.
I think it's strange that Erudite has to take over the Dauntless. If they are so smart, why don't they think up something with their own minds instead of taking control of other people's minds?