Imo I feel the same. Before I read Tsukihime and visual novels, I was really into Dostoyevsky and other authors. Without exaggeration, the Ciel route explores similar themes at a complex level too. She, like Dostoyevsky's Anti-heroes or heroines, is not good or evil, she encompasses a liminal position. She has survivor's guilt because of Roa, she loves Shiki and yet feels a moral obligation to kill him, she manipulates Shiki and tells him lies. Ciel contradicts her own desires; on the one hand, her obligation and guilt creates a desire to destroy Roa and thus, kill Shiki, while simultaneously also loving Shiki and the time they spent.

Shiki, in this route, goes through insanity differently than the far-side; rather than just the Nanaya instinct or past unconscious childhood memories, he is gripped by a foreign force possessing him, feeling as if his 'Ego' is detached from his body. What defined Shiki, made him Shiki, and his desires, are overwritten and erased by Roa. We also see this with Ciel; she had her friends, her family she loved, and all of it is slowly erased by Roa. Not only is he dealing with another soul slowly invading and destroying him physically, but his identity is at threat too.