The concept of a berserker has been a fascinating one to me since I learned about them as a kid. In videogames/RPGs, I tend to pick the close combat brute. Axes, especially, have always been my favorite armament.
When I bought 40k stuff as a kid, I chose Blood Angels and Space Wolves. Still the "gotta be the good guys" phase. Both armies excel at close combat, but they still have the loyalty-to-the-Emperor-and-moderating-your-emotions/motives-type-thing going on, which is kind of a drag when you're trying to be a villain.
Now the gods of Chaos are heavily involved with emotion. What greater emotion to feed on than anger in battle? While screaming "BLOOD FOR THE BLOOD GOD! SKULLS FOR THE SKULL THRONE!" is a rather poor battlecry, I think Kharn the Betrayer has a much more simple and effective chant in "KILL! MAIM! BURN!"
It isn't necessarily about any complex goals, simply kill to triumph over others and prove your superiority, all the while paying tribute to the bloodthirstiest patron imaginable.
The logical mind has largely left the building, leaving in its place a monster wholly consumed with the act of killing. The Chaos Marine codex points out that while devotees of Khorne might lose themselves in the midst of the carnage, they're still sound of mind between battles and such. From a storytelling standpoint, a Chaos Lord better be able to form a solid battle plan and avoid being lured into traps. Warriors still have to train and be competent soldiers, as being a champion of the Blood God demands skill with a blade of some sort.
I have a large contingent of World Eaters in my CSM army. I love Berzerkers, using a Khorne Lord on a Juggernaut as my warlord, and generally trying to have a superior close combat army.
At first, all I wanted to play was a Khorne-fluffy army, but over time I've come to appreciate the other gods as well as ranged units/psykers. Close combat needs support, smart deployment, and a bit of luck. It's also nice to have a versatile range of units to use in an army, so that I can have a good shot at beating any opponent if I pick the right list.