I play Chaos Marines. But I didn't start with them.

My first army was the Ultramarines, a faction that I played because my friend who introduced me to the game also played, and encouraged me to join him. It was also the most economical at the time, due to the AOBR box set costing only £40 (!!) at the time.
Anyways. I built myself an army of Ultramarines, including a Land Raider Crusader, and a lot of Tactical Marines. I enjoyed the army quite a bit. I would absolutely steamroll any horde/foot army I met, due to the crusader, and my tactical marines bringing the free missile launcher and flamethrower upgrades. After a while, I started noticing my army was kinda lacking in flavour, and I had gotten to know the vanilla marine codex so well that I could practically read it with my eyes closed. So I started looking at other things from GW.

Sure enough, there were some much better looking models around. I was intrigued, but I was an Ultramarine at heart. So I continued.
As 5th edition went on, more and more MEQ/Mech armies were appearing, and out of all the gamers that I knew, only one person played with a substantial number of troops. And I could very rarely get a game against him.
So it came to be that my army would get ROFL-stomped 9 times out of 10, and that other one time was a close loss. Lacking the money to improve my forces to tackle all the armour effectively, I tried to change tact with my army.

As it turns out, unless you have a wide variety of units/models from the vanilla codex, it's very hard to change they way your army plays. And that was something I didn't particularly enjoy.
I'm the kind of person who can get bored rather quickly, and this was beginning to wear my patience thin.

Looking into the fluff of other things in the 40k universe, I started to notice a lot of depth in the universe where grimdark, humans and bad-guys were concerned, but whenever I met a Space Marine in the fluff, it always seemed to read as follows:
"Space Marine murders bunch of aliens. Gets shot at by uber-powerful-weapon-that-should-have-killed-him but survived through sheer willpower/faith in the emprah".
They always seemed kinda flat, and in many cases, mary sue-ish.

It was around here that I started seeing Ultramarines (and the other loyal marines) absolutely everywhere that had anything to do with 40k. On the covers of books, battle reports, new releases, all GW stores I went into, even in pictures of other armies there were Ultramarines. I was starting to get sick of that damnable blue colour. And the fact that all the models seemed almost exactly the same (Tank = box with some sticks on it for guns. Trooper = faceless metal man. Leader = faceless metal man. Elite choice = faceless metal man. Fast attack = faceless metal man in a box with wings. Or sitting on a motorbike). There wasn't much for my imagination to go on anymore.

Further than that, I started to get an overload of pro-IOM every time I went near the hobby. Which made me start to hate it, because repetitiveness gets old quickly with me.

After this, I decided to take a break from the hobby, and learn how to paint (With a Daemon Prince, because it seemed one of the easier options to paint on as it has a huge surface [it's the plastic one], and looked excellent [The angular spikiness attracted me]).

I did a reasonable job, considering it was my first reasonable attempt at properly painting something. I used the colours I had around, ofc, and ended up with a Daemon Prince who wore boltgun-metal power armour, with gold trim, and had blue flesh (and glowing yellow eyes). It being my first reasonable paintjob, I decided to name it. Completely out of ideas for names, I decided to go with something from a song I had been listening to while painting him.
This song: http://www.youtube.com/watch?v=kzWUCiypeGA

I took the word "Sixteen" and converted it to latin. And thus named my Daemon "Sedecim".

It turns out that when you paint and name a Daemon Prince, and then place it on your desk opposite your bed, facing you as you sleep [creepy, right?], it starts to get stuck in your brain.

It was around this time that I started watching Miniwargaming's videos, especially "Slaughter's Rejects". I loved Dave's enthusiasm for chaos, and midway through one of his chaos tactica videos, I looked over at my Daemon Prince, and thought to myself, "Let's get a better look into this".

Sticking my head around in various places, I gathered as much knowledge as I could about the fluff for chaos, the army's gameplay, and some character background.
I had found a faction that had: insane blood crazed maniacs, nigh-indestructible automatonic warriors who fire MEQ-massacring bolts, fun-loving disease-ridden badasses who could laugh off wounds that could have felled even the mightiest hero of the Imperium, madness-inflicting horrors from beyond reality, and the ability to kit out everything for any situation.
I loved it.

So, with Sedecim at my side, I set about getting back at the IOM for the frustration and boredom it had inflicted upon me. The first act was to convert my Ultras to chaos. Not hard, really. The fluff is that Sedecim corrupted an entire company of Ultramarines (LOL), and nicked a good portion of the armoury, too. I found some new paints, and newbishly covered the majority of the marines in the colours of Nurgle, and some in the colours of Sedecim (they love Tzeentch). The painting worked as intended, firstly separating my marines from the Ultras, and then slowly adding chaos models and conversions to them (still a WIP), to make them chaossy.
I stuck some lascannons on the old crusader, got myself a Predator Annihilator, and started kicking the IOM's ass.

And I never looked back!
(Thank you for taking your time to read this)

After starting the Horus Heresy my favorite character was Garviel Loken. I thought he died at the end of the third book. Later I found out that he was found alive and became a founding member of the Grey Knights. Now I have a Grey Knight detachment and a Black Legion army to beat up.