Who would want to join the forces of Chaos, the hordes of monsters that threaten humanity with enslavement and extinction? The simple answer is the mad, the desperate, the greedy and the sociopathic - these are the sorts of humans and demihumans who actively embrace the worship of the Chaos gods. The Chaos Gods offer rewards of sorts, but few sane people would accept the terrible price that goes with them. The Chaos gods have been known to offer power, fighting prowess, forbidden magical lore, influence over others and wealth, but in exchange for their new followers' souls and even their humanity.
The worship of Chaos gods was generally outlawed in civilised lands before the Wars between the Empires. Today it is still outlawed where there is any law, but most of the world of Kaelaross has fallen into ruin and anarchy - not only is there no authority to stamp out chaos worship, but the very desolation and collapse of lawful civilisation has encouraged and hastened the re-emergence and rise of Chaos cults.
Chaos cultists usually follow one of the deities of Chaos, but there are some who worship Chaos as a whole philosophy and do not focus on any one specific deity. The philosophy of each chaos god will often determine how their followers operate.
Since they are outlawed, the chaos cultists practice their degenerate worship either in secret within civilised lands, or in the ruins or wilderness far from lawful authority. Those who operate within civilised lands can be very subtle and cunning, quietly extending their influence and recruiting members within towns and cities. Those who dwell outside of civilization will act as barbaric warlords where might makes right, the strong prosper and the weak are enslaved or killed.
The Red Hand is a prime example of a chaos cult within civilisation. It has several branches within the Kingdom of Teiglin that are mentioned in the descriptions of towns and baronies, but there are probably other branches and agents not mentioned. Somewhere in between the two types is White Stag Keep - the chaos cultists of Havoc there infiltrated a legitimate order of clerics and then overthrew them, becoming open worshipers of chaos in the process. They now control the keep and are too far away from the forces of Law to be ousted.
The relations between chaos cultists and the chaotic creatures that came through the Chaos Portals during the Summoning is very variable. Sometimes the monsters will attack and kill cultists as they would with any other humans or demihumans. At other times arrangements or bargains can be made - intelligent creatures will act as guards or enforcers in return for wealth or food. Chaos Cultists are sometimes dominated by powerful creatures of Chaos (such as vampires, medusae, chaotic dragons and fire or frost giants) who treat them as expendable slaves. Occasionally a chaos god will grant their followers, particularly the leaders of cults, control (or at least influence) over useful chaotic creatures.
Chaos cults always involve one or more cleric - like other deities, the Chaos gods appoint clerics as their emmissaries and intermediaries to mortals. These clerics of chaos are occasionally granted special spells that they can learn in place of standard clerical spells, but this is on a case-by-case basis (so chaotic cleric player characters should not automatically expect these spells). Other classes are found as well - fighters, magic-users, thieves and demihumans can all serve the gods of Chaos one way or another. Magic-users are sometimes guided towards grimoires of foul magics, and this can be a powerful lure to bring otherwise sensible wizards under the influence of Chaos - ambition and power can overrule common sense and decency in more magic-users than you might think. Thieves are, by their nature, unlikely to care for law or order - a chaos cult may well offer a thief a refuge from zealous watchmen and the cult won't care that the thief had just killed someone in a robbery - they will still offer healing and support in return for the thief's quiet loyalty. Fighters can be persuaded to aid the forces of Chaos with the offer of chaotically aligned magical weapons and armour that will make the fighter an unstoppable warrior. And many fighters acquire a taste for killing, and the chaos cult offers them a motive to kill.