That is what the warp is. It's the basest of emotions.
Everytime a sister of battle incinerates a heretic her hatred of them fuels the warp, every person who experiences emotion fuels it.
All this talk about overcoming and making better is foolish, at the end of the day everything we do is driven by the most base of instincts.
Yup, maybe, but it's tempered and channeled by intellect.
Chaos is just about being unrestrained about that, and not giving a damn about the other people. Surely, those thing exist in the imperium, but it's not the cardinal rules there. Now, show me a chaos corrupted chapter that treat their servitor as well as the Salamander, who are as noble as the Ultramarine, as dutiful as the Imperial Fist and maybe I will begin to consider chaos as a viable alternative.
Chaos isn't naturally present in the galaxy, the eye of terror is a cosmic accident created by an ancient psychic race that wouldn't be really bothering that many people if it weren't for that accident.
All of the chaos space marines chapters have become caricature of what they were before, without any real restraint or control, nor any real freedom other than to choose who they will destroy, plunder or attack in the name of their gods and the cultist aren't better, either.
Chaos, chaos god and chaos daemons are parasitic cosmic entity that feed on humanity, no more, feeding on their soul by promising power and freedom, when it truth, it give them nothing but eternal slavery.