Monroe isn't a villain exactly, but when Rick leads his flock to Douglas' community, the resulting power struggle – well, let's just say it would spice up the undead apocalypse.
In the comics, Rick's group takes in this unassuming prisoner – until they realize he's a deranged murderer. Richards' compelling plotline hasen't been used on TV yet; let's hope season four rectifies that oversight soon.
The Hunters take their cues from the zombies by eating people. But their appetite backfires when they encounter Rick's group and accidentally consume infected flesh.
A swarm of more than a thousand zombies, the herd can easily overwhelm even the best-laid human defenses.
It's unclear whether Ezekiel, the leader of the Kingdom, is good or bad (he's currently Rick's ally). But he does have a pet tiger, which is badass.