system
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human
[Warhammer 40k]So, if Chaos had the power to spirit all the Primarchs away, why not just kill them then?

Or raise them in the Warp, or put them in lava planets, or literally anything short of putting them with other humans for the Emperor to find?
gpt
This is what the Chaos Gods wanted. What better way to get at your greatest enemy than through his own "family"? They couldn't have raised them in the Warp, because they would have simply become daemon princes, which Big E can handle. They didn't want to simply kill the Primarchs, because then they don't have an "in".

What they did was allowed the Primarchs to be raised as Humans. As regular people. They were allowed to grow in an environment that wasn't being strictly regulated by someone like The Emperor, which is an environment that would have greatly lessened the chance of Chaotic corruption later down the line.

By the time Big E started finding them they were adults, and so had not had their minds barred against the influences of Chaos. Were they less susceptible than your average person? Totally. But it only takes one to start the domino effect. Really, the Chaos Gods got SUPER lucky with Horus being the one that started everything, because he was the favorite son. He was arguably the most powerful of all of the Primarchs and was generally respected by them all. 

Consider that the only reason that Horus was able to fell The Emperor is because he was holding back. He didn't want to hurt Horus. He didn't want to believe that his favorite son had turned to the very thing they were most against. In no other situation could they have ever gotten The Emperor into such a position. There is no other scenario that leads to his being mortally wounded without a favored Primarch being involved, because in no other situation does The Emperor willingly let down his guard. 

The Chaos Gods took a calculated gamble, and it turned out marvelously for them.