Most of these OSTs are 'cute'. See, Castlevania is not a horror franchise, it's spooky. So it's more of a cliche holloween theme but mature. Castlevania has mostly spooky soundtrack. And even when it has a horror, distorted soundtrack. The visuals in the series contradict it otherwise. But here are a few exceptions...
Etude For The Killer is creepy. Especially the Chronicles version.
This! The backgrounds with corpses hanging, some of them are dismembered by half, one is even skinned. Some corpses are butchered in a lab table. There were demon-babies in jar.
And my favourite, the last part of the stage gives you the sense of freedom as you walk out of those stairs and see beautiful mountains and daylight after the horror you went through... But as you walk, the backgrounds rips, only to find that you were fooled. It was nothing but a mare illusion to break you and give you that false sense of freedom. You're still trapped in hell. If it wasnt for a Belmont, an average person would've screamed in trauma just to find that you aren't free yet...
http://m.youtube.com/watch?v=HOLelQ-KC6sThis is pretty good. It gives you that dark ambience and gives you the sense of darkness before you even start the game. It's an ambient soundtrack so it only gives you the sense of horror. There is some vocalist in the background who chants some weird shit...
http://m.youtube.com/watch?feature=plpp&v=Rrp1ftrsq0QLoS too, had some soundtracks that were orchestrated to give some genuine sense of horror. This one in perticular did make me feel that you're now venturing off to really hellish ground. But alas, the visuals don't give off anything relatively scary. Castlevania will never be a scary game because as long as you play as superhuman-like warriors, it won't. As you're stronger, you fear less, and that's how the games make you feel. You're now seeing a Zombie right? Whip it, an it's gone. Because you're playing as powerful characters, bunch of zombies and skeletons won't fear you. Now add silent hill characters in the game, and now you can see that Dracula's Castle is a "House of Horror". You'd be a fool if an average human such as yourselves carelessly step foot on his domain...