Actually, Chaos is the manifestation of the darkness and evil in the hearts of people. Dracula somehow took that in. Because of this it became one with his powers and because of that, it influences him. It's also why there is a stream of Chaos existing within the castle. It's also why Soma nearly turned evil after he defeat Graham in AoS. If he had completely absorbed it, he would have become the master of the castle. It's why he went to the Chaotic Realm. By going there he could fight Chaos and by doing so, cut off so that it no longer can influence him. It's also why if you lose that battle, Soma becomes ascends to the throne. Remember, at the end of AoS after you've beaten Chaos, Soma tells Mina that deep within him, Dracula's power sleeps. It is no longer evil, but it has changed his life forever. If you don't believe me, play AoS again.
Eh... Actually...you're kinda... wrong and correct. ...Ahem... The Castle itself is a "Creature of Chaos." the "darkness and evil" in people's hearts only began after the events of Dracula's Curse and CoD Dracula had cast a horrible spell that drove man to the brink of insanity, malice and bloodlust which in turn cause chaos, thus feeding the "Demon Castle" thus feeding "Lord Dracula." while he was in his sleeper agent state.
If you pay attention... In OoE we find out that Dracula's power can be separated from him (i.e Dominus; i.e the Power of Dominance.) and absorbed in to glyphs...
If you pay attention in PoR, again we find out that Dracula's power (The Castle itself...) can separated from him (I.e the Chaos.)
....1999 Battle...
...Dracula is removed from both Power of Dominance (his 'soul is removed from the body) and Chaos (i.e the Castle)
...In AoS - ...Chaos beckons it's true master... i.e. Soma.... Which "activates" Dominance... By separating Chaos from Dominance, Somaula is "Free" again.
In DoS - We find out that Chaos was too much for the retard (i.e the last boss) to handle thus turning him into a huge demon, thus turning the "Demon" itself into a flesh entity... Thus Soma kill's it...thus sealing the powers away again... -_-;