Yami no Hitokiri
The disease Kenshin and Kaoru died of in Seisuckhen was symbolic of the self-loathing eating Kenshin alive from the inside. After he gave up the Sakabatou and his only means of atoning, he was overwhelmed by the self-loathing and eventually died after sharing it with Kaoru.
That's so sad...
I dont know exactly what it was but all I know is that it grew around his wrist and neck. 
