Marlow was saying that he wasn't really surprised although, Conrad seemed to show that the white men were as bad as or even more savage than the African counterpart. It is likely to be a total shift in morality. If he had had a lack of restraint then it should have left a while back.
Marlow also didn't seem to want to go against the rumors of Kurtz. Conrad on the other hand, is interested in keeping the rumor mill going, for whatever reasons. Marlow also had become a reliable narrator but he somehow was still biased towards Kurtz to becoming an absolutely reliable narrator.
