Character Analysis

If Eric had a soundtrack, it would mostly be this song. If most of the characters in the book serve as foils for Tris (because, uh, it's her book), Eric serves as a foil for Four: they are similar in some ways and radically different in others:

EricVs.Four
Piercings—lots of them (7.45)Piercings or tattoos?Tattoos (some hidden)

"the coldness of his eyes" makes him look menacing (7.45)

Their Eyes Have ItEyes have a "dreaming, sleeping, waiting color" (6.76)
"He stares with greedy eyes at Will's fallen body, like the sight is a meal and he hasn't eaten in weeks." (9.44)Reaction to People in Pain?Tris getting beaten up "wasn't something I wanted to watch" (11.89)
"For the first time, I recognize Eric for what he is: an Erudite disguised as a Dauntless…" (28.122)
Dauntless or Divergent?Wants to belong to all factions: "I want to be brave, and selfless, and smart, and kind, and honest." (31.75)
Mostly seems interested in punishing them, by, for example, throwing knives at Al (13) or making Christina hang from the chasm (9)TeachingYes, in that he gives pointers to Tris on fighting
Absolutely. He can be maniacal—but that might just be an act to hide how "perfectly controlled" he is (28.121)Two-faced?Yeppers. He looks intimidating (8), but is really a nice guy, as shown by his helping out Tris and Shauna (17.41) (Note: niceness does not apply when dealing with Peter.)

To put it another way, we could say that Eric is everything that's wrong with the factions: he's not just interested in knowledge (as Erudite should be)—he's interested in how knowledge can make him more powerful; and he's not just about being brave to help people (as Dauntless should be)—he's interested in cruelty for cruelty's sake.

Luckily for us, the book is not subtle about Eric being a bad dude. Tris will come right out and tell us that, for instance, "The curl of his lip is cruel" (9.44). There's not a lot of ambiguity there. And this is one opinion that Tris never needs to correct. He is just what he appears to be: a dangerous guy who doesn't care about other people's pain.