We're going to give Larkin his own page here because his betrayal of Lena and his family is worth examining. He helps Lena escape Macon's watchful eye and go to the surprise party, which, as it turns out, is really a trap. And after the party, he leads Lena right to Sarafine, pretty much putting her endgame in motion. Why so sneaky, Larkin?
"Loyalty's not really my thing" (2.11.84), Larkin says to Lena. Just like Ridley, this guy has no expression of remorse. But unlike Ridley, we don't see any glimmer of Light within him.
But really, was his betrayal a shock? The dude loves turning himself into a snake and flashing his yellow snake eyes. Sure, he can do wonderful things with his tongue (just ask Emily Asher), but that's the only positive thing he has going for him. If anything, Lena and Macon should have seen this coming from a mile away.
So why did he do it? It's simple, apparently: "It's a whole lot more fun being Dark than Light" (2.11.86). We'd love to say that his evil actions have consequences, but so far, no luck: he escapes with Sarafine and Hunting when Lena goes Unclaimed.