First, nobody tells you that it's a Chaos cult. It's not like you join today and tomorrow they have you sacrificing people to Tzeentch. Nobody says "hey, come join my evil cult!" It doesn't work that way.
Generally what will happen is that you are invited to join some sort of club or organization. On the surface, it looks fine. Maybe you're a cop or a soldier, and you get invited to be part of a "men's club" where you go drinking after work. It's the hardcore guys, the ones who seem to really be into their jobs. They do jiu-jitsu in their spare time, spend a lot of time at the gun range, work out a few hours every day, and probably take some steroids too. There's definitely a bit of over-the-top aggression, but hey it's an aggressive job.
One day you get partnered up with Vic and Tony, and when some rioter throws a brick at your face, Vic cracks the guy's skull open with a lead pipe. Then he hits the guy a couple more times. "You gotta do what you gotta do, man." Hey, Vic just saved your life. He's right, these people are animals. You should hang out with the crew a little more after work. "We need eight people for the group," they say. They don't mention why.
After a few months, you're shaving your head like the other guys. You've put on 25 lbs of muscle and are pretty heavy into martial arts and combat training now. The gang has started training with axes, because they think it looks badass. You start wearing t-shirts with skulls emblazoned on them underneath your uniforms and body armor, like you're part of a top-secret squad. Police brutality is generally approved in the Imperium, but you guys have decided to take it to a new level. Crime starts to drop in your neighborhood, and officers from other precincts ask how you guys managed to do it. Vic travels around, showing the other precincts how to set up their own "skull squads" and hand-picking officers who would best do the job.
You don't tell anybody about the unofficial raids you guys conduct a few times a week, where your 8 man skull squad goes out and ambushes some gang members in alleyways. You chop them up with axes and then dump the body parts in the industrial waste pipe. These guys are industrial waste anyway, real scumbags who got what they had coming to them (and the truth is, a lot of them really really are).
This has been going on for a while when Vic suggests you start adding some things to your look. You each get a cool 8-pointed star tattoo on your biceps. It looks pretty badass, like it's made out of barbed wire and spikes or something. You're still attending the required church services every morning before your shift, but you kinda agree with Vic that the Emperor is really more of a war god. "The Emperor protects" should really be "The Emperor demands blood". That's a lot more kickass. You start picturing the Emperor with giant wings and a big bloody axe.
So a few years go by. The neighborhoods where the skull squad teams have been introduced have seen lower rates of crime, but disappearances are way up. At first it was among real scum (rapists, murderers, etc), but now it's just higher in general. Some guy with three overdue parking tickets disappeared a few days ago, and no one has seen him since. Vic and Tony and the other guys are talking about how maybe you don't need to be cops anymore. Why are you still showing up to work? All those regulations and written reports just get in the way of justice.
Man, have you looked at Vic lately? You could swear that his eyes were glowing red last night. Like literally glowing red. You guys had a ritual facial scarring ceremony this weekend, to show your dedication to the cause. No turning back now. Some of the guys are filing their teeth into points, but you haven't done that yet. You are a little worried because last night some gangbanger shot you in the shoulder, but it didn't go that deep and you can swear it bounced off of something... something like metal under your skin. You're happy you're not dead and everything, but in the back of your mind you kinda know that's not normal biology.
Then one day you hear that the Hive City has been attacked by some sort of pirate force. They're apparently causing a lot of carnage and killing people. The police force called you up to find out where you've been, they need to deploy you to guard the industrial sector from attack. You and the other guys are sitting there in the dark, sharpening your axes. As you look around you count seven pairs of glowing red eyes, and in the gleam of your blade you can see the reflection of your own. You've been in contact with the other skull squads from the other precincts. You'd love to get in on this fight, the only question is... on which side?
