I am however wondering the cause of this all, where and how the choas started. That's one of my other dreading questions
I can give you a quick run-down in regards to the origin of chaos. Not spoilers, don't worry, just general background information.
"Chaos", or rather the Warp, has always existed as another dimensional layer skimming "reality". The warp is affected and stimulated by emotions, running through it like water in a stream.
Over the centuries as life spread across the galaxy , their emotions grew in quantity and power. Eventually, certain emotions grew so powerful, that their presence in the warp formed entities that could act independently. Over time, the three largest entities gained sentience:
Tzeentch: Embodiment of the volatility and vitality of change, learning, hope, scheming, and manipulation.
Khorne: Embodiment of emotions that lead to violence (hate, anger), the desire to slay one's foes, as well honour, pride, and general martial capability.
Nurgle: Embodiment of life and death, decay and persistence, perseverance and depression, comfort and suffering, and general stasis of all things.
Slaanesh: Embodiment of lust, greed, excess, pain, pleasure, perfection and hedonism - and thus very strongly related to teh arts.
It's important to understand that the Chaos gods aren't exactly evil, since they embody both positive and negative emotions - but their entire existence is based on sustaining themselves on the emotions that they need to survive, and as such scheme to encourage the presence of their respective emotional spectrum as much as possible within the galaxy, which by extension results in considerable damage. It's through this that they reach into mortal minds and encourage them into acts that will result in emotions that will empower them -be it slaughter, scheming, lust, or suffering. The destruction of the Imperium is secondary to them, after the emotions it would generate for them to feed on.
As for the Chaos Daemons, they are effectively tiny shards of their patron.