Who was the most dangerous criminal Chicago P.D. ever faced? A look back at the toughest case

Chicago P.D. hit TV screens back in 2014 and just parked itself in the middle of NBC’s One Chicago universe. The show has got people hooked with a raw look at policing in a city that has seriously got layers. It centers on Sergeant Hank Voight, who is the king of tough love and questionable morals. Over the years, you have got the squad digging into everything: mobsters, dirty politicians, human trafficking rings, and more. And it never shies away from showing how brutal or straight-up chaotic that life can get.

What actually makes Chicago P.D. stand out from the million other cop shows is that it is not afraid to get into the gray area of things. Justice isn’t always black and white, and Voight’s crew is the living proof. This isn’t just about chasing bad guys; it’s about loyalty, betrayal, and figuring out how far you’d go for your team, even if that means bending the rules.

They juggle these long, twisty storylines with those classic whodunit episodes, so you get deep character arcs and plot twists all at once.

And Voight is the walking tension. He will do literally whatever it takes to keep his people safe. That’s led to some seriously gnarly cases. The criminals aren’t just big, scary men; they’re clever, slippery, and sometimes so deep in the system you almost want to root for them.

Chicago P.D. has tackled everything: serial killers, cartel bosses, sleazy politicians, and traffickers. And it never pretends police work is a walk in the park. The risks are massive, not just for the people they’re trying to protect, but for the cops themselves. You see characters’ lives get chewed up and spit out by the job, and no one comes out unscathed.

One thing you've got to respect is that Chicago P.D. actually tries to get the details right. Real Chicago backdrops, procedures that feel legit, and stories that tap into what’s going on in the world right now. Whether it’s a raid gone sideways, undercover drama, or fallout from an officer-involved shooting, the show is never afraid to poke at the tough questions about what policing looks like nowadays.

So, that big question: Who’s the nastiest, most dangerous villain the Intelligence Unit’s ever squared off against? To even try to answer that, you've got to dig through all the Chicago P.D. rogues’ gallery from over the years.

It’s not just about who racked up the highest body count; it’s about which villain really got inside the squad’s heads, messed with their lives, and left scars that still sting. Let’s try to settle the debate on who was truly the worst villain on Chicago P.D.

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The toughest case on Chicago P.D.: The Sean O’Neal human trafficking ring

Sean O’Neal was a straight-up nightmare. Few villains on Chicago P.D. have ever crawled under our skin like he did. It wasn’t just the awful stuff he did, though that was bad enough—it’s how he wormed his way into everyone’s good graces. Cops, neighbors, whoever. He was like a wolf dressed as your friendly neighborhood golden retriever.

When he rolled in during Chicago P.D. Season 10, you could feel the whole vibe of the show get darker, more intense. Suddenly, it wasn’t only about chasing down the next bad guy; now you’re dealing with this twisted mastermind who messes with people’s heads, turns trust into a weapon.

The Intelligence Unit had to deal with way more than just bullets flying; they were fighting someone who made you question if you could trust anyone at all.

Sean O’Neal is the son of Chicago Police Chief Patrick O’Neal. He acts like he is some sort of savior, but peel back that shiny layer, and the guy is a snake.

Turns out, behind the scenes, Sean’s running a full-blown sex trafficking ring. He’s using his dad’s badge and reputation as a shield so he can hunt for vulnerable girls right under everyone’s nose.

So, Sean O’Neal pops up in Chicago P.D. Season 10, Episode 5. He crashes into the plot when Detective Hailey Upton finds this poor girl, Abby, all beaten up and chained to a bed. The Intelligence Unit jumps right into the mess, and then Sean O’Neal starts sniffing around the investigation. He acts all concerned, saying he's just looking for Abby because she goes to his youth center.

At first, he seems super helpful, almost too eager, but Upton has got her cop radar buzzing. The more Sean O’Neal hangs around, the weirder it feels. He has got these deep connections in the rougher parts of town, plus his whole weird vibe with the police.

So, as Upton digs deeper, she stumbles on some seriously messed up stuff as it turns out Sean O’Neal isn’t just tangled up in the trafficking ring, he is running the whole show. And he is using his so-called “support group” as a creepy recruiting ground to pull girls in.

The worst part is that Sean’s dad is literally the chief of police. The whole Intelligence Unit has to tiptoe around it, because going after Sean O’Neal could blow up their own jobs and drag the department’s reputation through the mud.

Jefferson White absolutely kills it as Sean O’Neal. The guy has got this weird energy like, you want to trust him, but at the same time, you’re side-eyeing every word out of his mouth.

Gwen Sigan (the showrunner), in conversation with CinemaBlend, even said he brought this “cult-leader” persona, which checks out.

Funny thing is, the character was supposed to be something else entirely. But White just rolls in, and Sean’s not your average villain. The scary part is he is so good at playing Mr. Nice Guy with a dark past, but under the surface, he’s twisting people around his finger.

The heat on Sean O’Neal just keeps escalating episode after episode. Upton is running on gut feelings at this point, and she spends weeks stalking him, barely sleeping, until she finally catches him slipping cash to some shady character everybody has already got their eye on.

That’s the golden ticket for the squad as they jump at the chance to go undercover, sweating bullets the whole time, trying to keep Chief O’Neal in the dark. But, of course, secrets never stay buried on TV.

The chief eventually figures out his own kid is at the center of the mess, and things explode. There is a screaming match, all the family drama you’d expect, and it goes to a pretty dark place.

Midseason finale hits, and the Intelligence Unit finally nails Sean O’Neal. They catch him red-handed: van packed with missing underage girls, no excuses left. The trafficking ring has got his name stamped all over it. But nothing ever goes smoothly in this show.

Chaos breaks out during the arrest, Chief O’Neal ends up dead, and Sean O’Neal is clinging to life after catching a bullet. Upton and Voight end up saving him.

Showrunner Gwen Sigan described Sean as “the most irredeemable person we could create,” while speaking with NBC Insider.

Even after they slap the cuffs on Sean, the guy just won’t fade into the background. He survives—somehow, and his shadow keeps creeping over the whole show. Everything he did left bruises that don’t heal, and the whole department is still dealing with the aftermath.

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Other notorious criminals

Sean O’Neal is a wild ride, but Chicago P.D. has thrown some gnarly villains at the team over the years. That Melissa Wilds' murder was a straight-up nightmare fuel. Barbed wire, mutilation, just pure horror. Cases like that crank up the body count and make everyone sweat, but they don’t always hit as hard on a personal level as the O’Neal drama did.

Now, if we’re talking about real snakes in the grass, Denny Woods takes the cake. The guy was supposed to be one of their own—a former lieutenant. Instead, he turns around and starts playing puppet master, blackmailing, manipulating, and bending the rules to trash Voight and his squad.

And let’s not forget, his scheming primarily put Olinsky in a coffin. Woods wasn’t butchering people like the cartel psychos, but the way he twisted the system from the inside, that’s a whole different kind of dangerous.

And then, you’ve got those episodes digging into wrongful convictions like the John Fulton and Anthony Mitchell case. Guys rotting in prison for 16 years for a crime they didn’t commit. It’s just plain old system failure. Still, it hits you—sometimes the justice system itself is the real villain.

Chicago P.D. isn’t scared about throwing the Intelligence Unit into the deep end—whether it’s sicko serial killers, backstabbing cops, or the broken system itself. Keeps you on your toes, that’s for sure.