Originally Posted by
CoffeeGrunt
As Yorkie said, if your situation is desperate enough you will cling to anything.
The Ruinous Powers tempt in a hundred, thousand different ways. They'll find your weakness and open that baby up, they'll make you dance on their strings just so you can cover or remove that weakness or turn it into a strength. Horus wanted power and they promised him it. They took his soul and gave him unimaginable power, the ability to crush a galaxy-spanning empire at the peak of its strength.
But Magnus? Magnus was led to underestimate Tzeentch. He was adamant he had agency and mastery of his fate, but ultimately Tzeentch's vastly superior intellect knew and understood Magnus, knew the choices he would make, and what it would take to tempt him. Appeal to his ego, grant him great power, freeze the clock on the Flesh Change. Tzeentch knew that Magnus was egotistical and arrogant enough to go do exactly as he wanted him to, provided he approached Magnus as a lesser daemon to be mastered, rather than a vastly superior mind Magnus still cannot comprehend. That right there. Magnus, one of the Primarchs, apex of the human race, with ten thousand years of immortal time and an unmatched intellect and education, cannot understand Tzeentch.
It's suggested even that Fateweaver, Tzeentch's own right-hand Daemon, is trying to subvert Tzeentch and guide him down the wrong path after Tzeentch threw him into the deepest pit of the Warp to ply the future. His followers are bought and discarded whenever his long plan demands it, and while you feel your role is to conquer worlds in his name, Tzeentch knows he'll take you right to the penultimate battle, and abandon you and your forces to die, because in defeating you the Imperium reclaims the territory, and a certain person will stumble upon a Chaos relic that might sway them, allowing him to corrupt the entire army that slew you in their turn. Thus Change cycles anew.
As far as Khorne? Blood must flow. Bloodletters and World Eaters would quite happily rip each other to shreds once their mutual enemy is slain, because the blood must flow. It must always flow.
Slaanesh? Who cares who's being maimed, tortured or pleased, so long as it keeps going. Their soldiers are twitching, frothing addicts to their senses, needing that next hit, no matter where or what feeds it.
So, you ask, why do people go into it? For the same reason people scoff at the homeless and keep running in the rat race we live in today. A Chaos Marine looks at the bloodthirsty Khorne marine, the ensnared Tzeentch cultist, the Slaaneshi sense-addict or the Nurgle bag of pus, and thinks:
"It'll never happen to me. My will is too strong, I'll make the Gods serve me!"
Yet each of them makes pacts, and steadily is tricked into surrendering their souls and becoming those people. Some do it gladly, some might not.
Yet, the only person to make that claim and succeed with it, is Abbadon. Abbadon isn't owned by a God, he owns them all. None of them lays claim to his soul, but they lavish him with power and riches. If the Gods are cruel lovers who abuse their followers, then Abaddon is the one who learned to turn that tactic back on them. He leads them on, accepts their gifts, and stays with them only as long as necessary to get what he wants while their objectives align.
And he's an example of what Chaos could do, if you were only strong enough to bend it to your will.
And of course you're strong enough...
...Right?