In book eleven. Ivan goes crazy and starts talking to the devil every night. Or, what he believes is the devil. We discover that Smerdyakov is actually the killer. He admits it to Ivan. Even though he thinks that Ivan already knew. Apparently, Ivan told him to kill Fyodor even though, Ivan doesn't remember doing it but, he admits to doing it anyways. It ends up that Smerdyakov hangs himself.
I think Ivan went more than crazy lol