Board Thread:Questions and Answers/@comment-26155961-20150228185909/@comment-62.183.154.102-20150228201029

He does it to re-experience the thrill he felt when he finally turned the tables on the investigator who had been torturing him.

Here's the chapter where he tells Kaneki his story: http://www.twistedhelscans.com/read/tokyo_ghoul/en/7/61/page/13