Strictly speaking, the Chaos Gods aren't exactly embodiments of negative/evil thoughts and emotions. It's more accurate to say that they're "pure" emotions, each representing a sort of emotional drive, purified to the point where it becomes a dangerous menace. They are what happens when people lose themselves to their emotions, because they ~are~ emotion without context or nuance.
Khorne is opposition. When someone feels hatred and rage towards a thing, feels the urge to destroy or oppose it, that is what creates and feeds Khorne. His hatred drives him to carve a bloody swath across the universe, leading armies to revel in the destruction of their enemies.
Slaanesh is devotion. When someone feels love or obsession towards anyone or anything, whether a lover, a sensation, or even an art form, that is what Slaanesh is made out of. He embodies the concept of wanting as much of something as you can have, bringing it to ever-encreasing heights while the things that once excited you seem dull by comparison.
Nurgle is resignation and contentment. He is the God of those who accept things as they are, who let nature take its course because they cannot change it. He is despair, stagnation, the festering plague, whose love for his followers allows them a morbid contentment which lets them find peace.
Tzeentch is desperate hope. Those who cannot accept things as they are, who plot to bring about change, draw his eye. He shows them something to strive for, makes deals and promises, but his plans always seem to turn against the planner and leave them wanting more; after all, anyone who is content would not need to hope so desperately.
The grim darkness of the galaxy does tend to add quite a bit of... nuance... to the Chaos Gods. But their malevolance is a result of what they are: emotional drives scattered in the Warp, coming together with similar drives, flowing in certain directions to become self-aware storms of power.