Hmm, I initially didn't think Koito would turn on Tsurumi, mostly because his whole character is mainly focused on the fact that he idolizes Tsurumi insanely and it would be too sharp of a turn for him to change his mind on that, but, as other people are pointing out, Koito doesn't look as genuine in his Tsurumi admiration at the end, so maybe he is faking it. I could still see it being real though- if he's forced to realize that the man he admires was a fake, it would be easier to trick himself into justifying Tsurumi's actions rather than acknowledging the truth.