Lot of people fall into the Slytherin=Baddie/Gryffindor=Heroes trap. Cunning and ambition are the 'prized' traits, not megalomania or psychosis. Yes most Death Eaters were slytherins, but not all syltherins were DEs. Daphne Greengrass and her friend Tracy Davis are slytherins, but not Death Eaters. Neither (as far as they're taken in the books anyways) are capable of murder. Now the High Profile Slytherins are another story, but as far as I could tell, a good portion of the house stay low and do their work.
As such, I think Kodachi would be a good fit in Ravenclaw, insanity aside, she's brilliant with poisons, and that takes more then a little bit of smarts. Or, to really mess with readers heads, put her in Gryffindor. She's certainly bold enough to fit the criteria. :p